Yes, VMSXX is tax-exempt at the federal level: dividends from the Vanguard Municipal Money Market Fund are exempt-interest dividends and owe no regular federal income tax.1Vanguard. VMSXX – Vanguard Municipal Money Market Fund That’s the short answer, and it’s also the part most people already know. The longer answer is that the federal exemption does not carry over to state income tax on out-of-state bonds, does not shield you from the alternative minimum tax if the fund holds private activity bonds, and does not keep the interest out of several federal calculations that can still raise your tax bill.
Why the Federal Exemption Applies
Under 26 U.S.C. §103, interest on bonds issued by state and local governments is excluded from gross income for federal tax purposes.2Office of the Law Revision Counsel. 26 USC 103 – Interest on State and Local Bonds VMSXX invests exclusively in short-term municipal obligations and passes that tax-exempt character through to shareholders as exempt-interest dividends.
The pass-through isn’t automatic for every mutual fund. A regulated investment company qualifies to pay exempt-interest dividends only if at least 50% of its total assets consist of tax-exempt municipal obligations at the close of each quarter.3Office of the Law Revision Counsel. 26 USC 852 – Taxation of Regulated Investment Companies and Their Shareholders VMSXX holds 100% of its assets in tax-exempt securities, so it clears that bar with room to spare.1Vanguard. VMSXX – Vanguard Municipal Money Market Fund
State Tax Is a Separate Question
Most states exempt interest only from bonds issued within their own borders and tax interest from out-of-state municipal bonds as ordinary income.4Municipal Securities Rulemaking Board. Municipal Bond Basics VMSXX holds a nationally diversified portfolio, so a significant portion of its dividends will typically come from bonds issued outside your home state and will be taxable on your state return.
How much that matters depends on where you live. Nine states impose no income tax, which makes the question moot. Elsewhere, top marginal rates range from 2.5% to 13.3%. Each year, Vanguard publishes a state-by-state breakdown showing what percentage of each fund’s income came from obligations issued in each state.5Vanguard. Tax-Exempt Interest Dividends by State for Vanguard Municipal Bond Funds Multiply that percentage by your total exempt-interest dividend to work out how much is exempt on your state return.
One narrow exception: interest from bonds issued by U.S. territories such as Puerto Rico, the U.S. Virgin Islands, and Guam is exempt from federal, state, and local income tax regardless of where you live.6Office of the Law Revision Counsel. 48 US Code 745 – Tax Exempt Bonds If VMSXX holds territorial obligations, Vanguard’s annual tax supplement identifies them.
Private Activity Bonds and the AMT
Not every municipal bond is treated the same for AMT purposes. Some of VMSXX’s holdings may be private activity bonds, which finance projects that serve partly private purposes such as airports, housing developments, or industrial facilities. Interest from these bonds is exempt from regular federal income tax but counts as a tax preference item that gets added back when calculating the alternative minimum tax.7Office of the Law Revision Counsel. 26 USC 57 – Items of Tax Preference The statute specifically extends this treatment to exempt-interest dividends paid by mutual funds, proportional to the fund’s holdings of such bonds.
For 2026, the AMT exemption is $90,100 for single filers and $140,200 for married couples filing jointly. Those exemptions begin to phase out at $500,000 and $1,000,000, respectively.8Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026 If your income plus preference items stays below the exemption, private activity bond interest won’t trigger extra tax. Near or above the phase-out range, even a modest amount can move the needle. Vanguard reports the AMT-subject percentage in its year-end tax information, which flows to Line 2g of Form 6251.9Internal Revenue Service. Instructions for Form 6251
Federal Calculations That Still Use Exempt Interest
Several federal calculations pull tax-exempt interest back in, even though you don’t owe income tax on it directly. This is where the “tax-exempt” label gets misleading.
Social Security Benefit Taxation
The IRS determines how much of your Social Security benefits are taxable by adding half your benefits, all of your other income, and all of your tax-exempt interest.10Internal Revenue Service. Social Security Income If that combined figure exceeds $25,000 for a single filer or $32,000 for a married couple filing jointly, a portion of your benefits becomes taxable. At higher levels, up to 85% can be taxed. A large VMSXX position generating meaningful exempt-interest dividends can push retirees across these thresholds and make benefits taxable that otherwise wouldn’t be.
Medicare IRMAA Surcharges
Medicare Part B and Part D premiums increase at higher income levels through Income-Related Monthly Adjustment Amounts, and the modified adjusted gross income used for IRMAA includes tax-exempt interest.11Social Security Administration. Medicare Income-Related Monthly Adjustment Amount – Life-Changing Event For 2026, individual filers with MAGI above $109,000 (or $218,000 for joint filers) pay progressively higher premiums, and the top-tier Part B surcharge reaches $487.00 per month.12Centers for Medicare and Medicaid Services. 2026 Medicare Parts A and B Premiums and Deductibles A retiree parking several hundred thousand dollars in VMSXX can generate enough exempt interest to cross an IRMAA threshold and add over $970 to annual Part B premiums without any corresponding rise in spendable income.
Interest You Paid to Buy the Shares
If you borrow money to buy or hold VMSXX shares, you cannot deduct the interest on that debt. Federal law disallows deductions for interest on debt incurred to purchase or carry tax-exempt obligations and explicitly extends the rule to shares of regulated investment companies that pay exempt-interest dividends.13Office of the Law Revision Counsel. 26 USC 265 – Expenses and Interest Relating to Tax-Exempt Income Investors using margin accounts or home equity lines to fund VMSXX purchases should assume the IRS will disallow the corresponding interest deduction.
Reporting VMSXX on Your Return
After each calendar year, Vanguard issues Form 1099-DIV summarizing your dividends and distributions. Tax-exempt interest dividends appear in Box 12.14Internal Revenue Service. Form 1099-DIV, Dividends and Distributions Even though the income isn’t taxed, the IRS requires you to report it on Line 2a of Form 1040.15Internal Revenue Service. Instructions for Schedule B (Form 1040) That figure feeds into the provisional income calculation for Social Security and the MAGI used for IRMAA.
If any portion of your dividends came from private activity bonds, Vanguard identifies the amount separately, and it goes on Line 2g of Form 6251.9Internal Revenue Service. Instructions for Form 6251 For your state return, use Vanguard’s state-by-state breakdown to work out how much of your exempt-interest dividend qualifies for exemption where you live.
Automated IRS matching compares what Vanguard reports on Form 1099-DIV against what appears on your return. Omitting exempt-interest income from Line 2a, or failing to include AMT-subject interest on Form 6251, can generate a notice. The numbers aren’t complicated; the mistake is assuming “tax-exempt” means “nothing to report.”