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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsShort answer: Neither category is automatically safer, better-paying, or more liquid. A Treasury bond fund mainly exposes you to changes in Treasury yields and the fund’s duration. A mortgage bond fund, generally a fund holding mortgage-backed securities (MBS), carries those interest-rate risks plus changing prepayment and extension behavior. Compare the individual fund’s duration, holdings, expenses, structure, trading terms, and current documents rather than relying on its category label.
This guide applies to U.S. funds and securities. It also explains why a bond fund can lose money even when its holdings have strong government backing, how falling and rising rates affect MBS differently, and what “liquidity” really means for an ETF, a mutual fund, and the securities inside the portfolio.
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What the two fund categories actually hold
Treasury bond funds
A Treasury bond fund pools investor money to buy U.S. Treasury bills, notes, bonds, and sometimes Treasury inflation-protected securities or Treasury futures. The category includes very different portfolios. A fund holding two-year notes behaves differently from one holding 20- or 30-year bonds, and an actively managed portfolio can change its exposure over time.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThe U.S. government’s payment backing addresses the credit of the Treasury securities; it does not lock the fund’s share price. If market yields rise, existing bonds with lower coupons generally become less valuable, and the fund’s net asset value (NAV) can fall. A long-duration Treasury fund can therefore be much more volatile than a short-duration fund, even though both are called Treasury funds.
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Mortgage bond funds
“Mortgage bond fund” is shorthand for a fund investing in mortgage-backed securities. The label is not a guarantee that every portfolio owns the same type of mortgage exposure. A fund may hold agency MBS with government-sponsored guarantees, other government-related securities, commercial mortgage securities, non-agency residential MBS, or a mix. Read the prospectus and shareholder report to identify the securities, guarantors, credit ratings, and concentration limits.
An MBS represents claims on pools of mortgage payments. Homeowners can repay their loans early, usually by refinancing, selling a property, or making extra principal payments. That homeowner behavior gives MBS a cash-flow pattern that can change when interest rates move.
The risks: where the categories differ
Interest-rate and duration risk
Duration is an estimate of how much a portfolio’s price may change for a given change in yields. It is not a promise, and it can shift as the portfolio and market assumptions change. A fund with an effective duration of six years would, as a rough starting point, be expected to lose about 6% if yields rose one percentage point, before considering convexity, credit effects, fees, and other factors. The actual result can differ materially.
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Check the fund’s current effective duration, maturity distribution, yield-curve exposure, and interest-rate hedges. Two funds with “Treasury” in their names can have completely different duration risk. The same is true for MBS funds, with the additional complication that expected mortgage prepayments can change duration.
Prepayment risk when rates fall
When mortgage rates decline, homeowners have more incentive to refinance. An MBS fund may receive principal sooner than expected. The manager then has to reinvest that money, often at lower prevailing yields. Faster repayments can also limit the price appreciation an MBS would otherwise gain when rates fall, because the high-coupon mortgage is likely to disappear sooner.
This is why MBS commonly offer higher yields than bonds with comparable credit quality and maturity: investors are being compensated, in part, for the possibility that borrowers will repay early. The yield premium is not a guarantee of higher total returns. It can be consumed by refinancing waves, falling reinvestment rates, trading costs, expenses, or an unfavorable rate path.
Extension risk when rates rise
When rates rise, refinancing becomes less attractive and homeowners tend to keep existing mortgages longer. Principal arrives more slowly, so the MBS can remain outstanding for longer than expected. Its effective duration extends, increasing sensitivity to additional rate increases. This is called extension risk.
Extension can make an MBS fund behave more like a longer-maturity bond precisely when rates are moving against it. A portfolio that appeared moderately interest-rate-sensitive in a low-rate environment may become substantially more sensitive after a sharp increase in mortgage rates.
Credit and guarantee risk
Credit backing and market-price risk are separate. Agency or government-supported MBS may have strong assurances regarding timely payment of principal and interest, but those assurances do not prevent NAV losses when market yields change. Non-agency MBS can add borrower-default, servicing, tranche, and structural risks. The prospectus should state what is guaranteed, by whom, and under what conditions.
Can either fund lose money?
Yes. A bond fund does not hold each bond for you until maturity. It continuously values its holdings at market prices, buys and sells securities, distributes income, and absorbs expenses. If rates rise, credit spreads widen, prepayments change, or trading conditions deteriorate, the NAV can decline. Selling shares below your purchase price creates a realized loss even if the fund later recovers.
An individual Treasury security held to maturity has a different cash-flow promise: assuming the issuer pays as scheduled, you receive the stated principal at maturity. A Treasury fund has no single maturity date for the portfolio, so it does not provide that same maturity-value certainty. Do not treat a Treasury fund as cash, a money-market fund, or a guaranteed stable-NAV product.
Returns: what can be compared fairly
Yield is not total return
A fund’s current yield is a snapshot of income under a stated calculation. Total return combines distributions and the change in NAV or market price over a specified period, after expenses. A fund can show a higher yield and still produce a lower total return if its NAV falls more, if prepayments force reinvestment at lower rates, or if it carries higher costs.
When comparing funds, record the date and definition of every yield figure. Then compare total returns for identical periods, with distributions reinvested on the same basis and the same share-class assumptions. A seven-day SEC yield, a trailing-12-month distribution yield, and a calendar-year total return answer different questions.
How the rate cycle can change the outcome
| Market situation | Typical Treasury-fund effect | Typical MBS-fund effect | Important qualification |
|---|---|---|---|
| Rates fall rapidly | Existing Treasuries can rise in price, with larger moves in longer-duration funds. | MBS may gain, but faster prepayments can cap appreciation and force reinvestment at lower yields. | Results depend on duration, mortgage coupons, spread movements, and how quickly borrowers refinance. |
| Rates rise rapidly | Prices generally fall; longer-duration funds usually fall more. | Prices can fall from rate increases, while slower prepayments extend duration and can deepen sensitivity. | Credit spreads, hedges, and the starting level of prepayment assumptions also matter. |
| Rates move sideways | Income and roll-down may be the main contributors, offset by expenses and trading. | Income may include an MBS yield premium, but realized cash flows depend on borrower behavior. | Stable rates do not ensure stable NAVs or positive returns. |
What the market-size figures do—and do not—show
The SEC Analytics Office reported that registered taxable bond funds held approximately $2.249 trillion in U.S. Treasury debt and $1.621 trillion in mortgage-backed securities in March 2026. These aggregate gross investment values show that both exposures are substantial in the U.S. fund market. They are not performance results, risk rankings, or evidence that one category is preferable.
There is no single category-wide winner. Outcomes depend on the yield available when the fund buys, duration, portfolio turnover, prepayment behavior, expenses, credit spreads, and the path—not merely the endpoint—of interest rates.
Liquidity: separate the fund shares from the holdings
ETF share liquidity
An ETF trades on an exchange during market hours. You can enter a market or limit order, but the execution price is determined by supply and demand and may differ from NAV. An ETF can trade at a premium (above NAV) or discount (below NAV), especially when the underlying securities are difficult to price or markets are stressed. Check average trading volume, quoted bid-ask spread, typical premium or discount, and the time of day you plan to trade.
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Exchange trading makes an ETF readily tradable; it does not guarantee a desirable price. A thinly traded ETF can cost more to enter or exit through a wide spread. A limit order can control the maximum purchase price or minimum sale price, but it may not execute.
Mutual-fund transaction mechanics
Mutual-fund shares are generally bought or redeemed at the next calculated NAV, rather than at an intraday exchange price. The fund’s prospectus states its cutoff time, redemption procedures, settlement cycle, and any fees or restrictions. A mutual fund may be easy to redeem under ordinary conditions while the underlying securities remain harder to sell or value during market stress.
Liquidity of the underlying bonds
Fund-share tradability and security-market liquidity are different. Treasuries usually trade in a deep market, but liquidity can vary by issue and market conditions. Some MBS pools trade actively; others can be more difficult to price because their cash flows depend on assumptions about prepayments. During stress, dealer capacity, bid-ask spreads, and price transparency can deteriorate. Read the fund’s liquidity-risk disclosures and the shareholder report’s description of less-liquid holdings.
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Direct Treasury purchases are a separate case
TreasuryDirect allows individuals to buy marketable Treasury securities directly, while banks, brokers, and dealers provide other purchase and resale routes. TreasuryDirect requires a $100 minimum bid. Newly purchased marketable securities generally have a 45-calendar-day hold before they can be transferred or sold through that platform. Securities can also be bought and sold in the commercial market after issuance, subject to that route’s pricing and transaction terms.
Those TreasuryDirect rules apply to individual securities, not to ETF or mutual-fund shares. Do not use the 45-day platform hold as a measure of a Treasury fund’s liquidity.
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A step-by-step comparison method
- Define the exposure. Confirm whether the Treasury fund owns bills, notes, bonds, inflation-protected securities, futures, or a mixture. For the MBS fund, identify agency, government-backed, or non-agency holdings and the percentage in each.
- Record duration and maturity. Write down effective duration, average maturity, maturity ranges, and any stated duration targets. Note whether the manager can change them substantially.
- Read the prepayment disclosures. For an MBS fund, look for assumptions about prepayment speeds, weighted-average loan age, coupon distribution, and extension scenarios. A duration figure without its assumptions is incomplete.
- Compare income measures on one date. Use the same yield definition and date for both funds. Keep current yield separate from historical distribution yield and from total return.
- Compare matched-period total returns. Use identical start and end dates, reinvestment assumptions, and share classes. Include periods of rising, falling, and stable rates when available.
- Subtract all costs. Check the expense ratio, acquired-fund fees if disclosed, sales loads, redemption fees, transaction charges, and the ETF’s bid-ask spread. A stated yield before expenses is not your net result.
- Assess trading terms. For an ETF, review volume, spread, and premium or discount to NAV. For a mutual fund, review the NAV cutoff, settlement, redemption terms, and any fees.
- Check tax information. Read the fund’s annual tax supplement and distribution notices. Do not assume that the tax treatment of a fund distribution exactly matches the treatment of interest from a Treasury security.
- Review current documents. Use the latest prospectus, summary prospectus, shareholder report, holdings file, and risk disclosures. Category names and older fact sheets can be stale.
Taxes and account placement
TreasuryDirect states that interest on Treasury bonds is subject to federal income tax but exempt from state and local income taxes. That statement concerns interest from Treasury bonds held directly. A fund may distribute interest, capital gains, return of capital, or income from securities other than Treasuries. Its tax reporting determines what you actually report.
For a Treasury or MBS fund, inspect the year-end tax supplement for the portion of distributions identified as U.S. government interest, the percentage eligible for state-tax exemption, and any capital-gain distributions. Rules can differ by state, and a fund’s qualifying percentage may change each year. In a taxable account, compare after-tax income rather than headline yield alone. In a tax-advantaged account, the state-tax distinction may be less relevant, but expenses and risk remain.
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- “Treasuries cannot lose money.” An individual Treasury held to maturity has a stated maturity payment if the issuer pays, but a Treasury fund’s NAV fluctuates daily. Check duration before assuming stability.
- “The higher-yielding fund is better.” Yield compensates for risks and may not survive prepayments, rising rates, defaults, or expenses. Compare after-expense total returns.
- “All mortgage securities are guaranteed.” Agency and non-agency MBS have different guarantees and credit risks. Verify the guarantor and the conditions of support.
- “An ETF always trades at NAV.” Exchange prices can be above or below NAV. Review the historical premium or discount and current spread.
- “Duration is fixed.” MBS duration can extend when rates rise and prepayments slow. Recheck the figure after major market moves.
- “A fund is as liquid as its most visible price.” The ability to click sell does not guarantee a narrow spread or a fair valuation for underlying holdings. Examine liquidity disclosures.
- “Past category returns predict the next cycle.” The same rate move can produce different results depending on starting yields, mortgage coupons, prepayment speeds, and portfolio positioning.
Which type may fit a particular objective?
When a Treasury fund may be the clearer choice
A Treasury fund may be easier to analyze when you want transparent government-security exposure and can tolerate the duration risk shown in its fact sheet. A short-duration portfolio can reduce—but not eliminate—price volatility relative to a long-duration portfolio. A long-duration portfolio may be intentionally chosen for a view that rates will fall or for diversification against other risks, but it can experience large losses when yields rise.
When an MBS fund may be appropriate
An MBS fund may suit an investor seeking diversified mortgage exposure and potentially higher income than a comparable-credit, comparable-maturity Treasury portfolio, while accepting prepayment and extension risk. The choice requires confidence that the fund’s structure, guarantees, duration management, and expenses fit the intended holding period.
When neither fund is a cash substitute
If the money must retain a stable value over a very short horizon, neither a Treasury bond fund nor an MBS fund should be treated as a guaranteed cash account. Match the fund’s duration and possible drawdown to the date you may need the money. A fund that is reasonable for a multi-year allocation can be unsuitable for a bill due next month.
Questions to answer before buying
- What is the fund’s effective duration today, and how could it change?
- What percentage is in Treasuries, agency MBS, government-backed MBS, non-agency MBS, cash, and derivatives?
- What assumptions about prepayments drive the reported duration and yield?
- What was the worst recent drawdown, and over what dates?
- Are the return figures net of expenses and based on the same share class?
- For an ETF, what are the normal bid-ask spread and premium or discount to NAV?
- For a mutual fund, when is the redemption order priced and settled?
- How are distributions classified for federal and state taxes?
- Does the current prospectus disclose risks that the fund name does not reveal?
Bottom line
Treasury bond funds generally offer simpler exposure to interest-rate and duration risk, while mortgage bond funds add the moving cash flows of homeowner prepayments. Falling rates can trigger refinancing and lower-yield reinvestment in MBS; rising rates can slow prepayments and extend duration. A higher MBS yield is compensation for those risks, not a performance promise. For either category, compare the specific fund’s current holdings, duration, expenses, return history, tax reporting, and trading mechanics before deciding.
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