Tax-exempt money market funds are mutual funds that hold short-term municipal debt so the interest they pay out is generally free from federal income tax. They aim to hold a steady $1.00 share price, offer daily liquidity, and pass through interest as exempt-interest dividends that skip the ordinary income tax most cash yields incur. For investors in higher tax brackets parking cash outside a retirement account, that combination can beat a taxable money market fund on an after-tax basis. Whether it actually does for you depends on your marginal rate and your state.
What’s Inside the Fund and Why the Income Escapes Federal Tax
The tax break comes from the underlying securities. These funds buy debt issued by state and local governments and their agencies to finance public projects. Section 103 of the Internal Revenue Code excludes interest on state and local bonds from gross income, subject to exceptions for private activity bonds and arbitrage bonds.1Office of the Law Revision Counsel. 26 USC 103 – Interest on State and Local Bonds When the fund collects that interest and distributes it to you, the tax-free character survives. The fund reports the payments as exempt-interest dividends in Box 12 of Form 1099-DIV.2Internal Revenue Service. Instructions for Form 1099-DIV Capital gains the fund realizes from selling securities remain taxable.
SEC Rule 2a-7 caps the fund’s weighted average maturity at 60 days and limits any single holding to 397 days.3eCFR. 17 CFR 270.2a-7 – Money Market Funds Long-dated municipal bonds cannot fit. The funds rely instead on short-term municipal notes issued for seasonal cash-flow needs and, more heavily, on Variable Rate Demand Notes. VRDNs are longer-dated municipal bonds with a put feature that lets the holder sell them back to a remarketing agent at par on a daily or weekly basis, which allows the fund to treat them as short-term for maturity purposes while capturing tax-exempt interest. Most tax-exempt money market fund portfolios are dominated by VRDNs.
Federal Exemption Is Automatic; State Tax Depends on Where the Bonds Are From
The federal exclusion under Section 103 applies regardless of which state issued the bond or where you live.1Office of the Law Revision Counsel. 26 USC 103 – Interest on State and Local Bonds State income tax works differently. Most states only exempt interest on bonds issued within their own borders, so a Massachusetts resident holding a fund with Texas-issued bonds will generally owe Massachusetts tax on that share of the interest.
That splits the category in two:
- National tax-exempt funds hold municipal debt from issuers across the country. You reliably avoid federal income tax, but most of the interest stays subject to your state’s income tax.
- Single-state funds concentrate holdings in one state’s municipal securities. If you live in that state, you get a double exemption: no federal income tax and no state or local income tax on the interest.
A high-income New York City resident using a New York-specific fund can shed federal, New York State, and New York City income taxes on the interest. The cost is concentration: a single-state fund’s credit quality depends on issuers within one state.
The AMT Wrinkle
Interest on certain private activity bonds, where the proceeds primarily benefit a private entity rather than the general public, counts as a tax preference item under the alternative minimum tax.4Municipal Securities Rulemaking Board. Tax Treatment5Internal Revenue Service. TEB Phase II – Lesson 4 General Rules for Private Activity Bonds That interest is still excluded from the regular federal income tax, but it gets added back when you calculate AMT liability. Most taxpayers clear the AMT exemption entirely, but if your income and deductions push you close to the AMT line, private activity bond interest inside your fund can partially erode the tax benefit you thought you were getting.
Some fund companies address this directly by offering AMT-free tax-exempt money market funds that screen out private activity bonds. If you have any reason to think AMT applies to you, an AMT-free fund removes the guesswork.
The NIIT Advantage for High Earners
The 3.8 percent Net Investment Income Tax applies to the lesser of your net investment income or the amount by which your modified adjusted gross income exceeds $200,000 (single) or $250,000 (married filing jointly). Tax-exempt municipal bond interest is excluded from net investment income for NIIT purposes and does not raise your MAGI for this calculation.6Internal Revenue Service. Topic No. 559, Net Investment Income Tax A taxable money market fund paying the same nominal rate would trigger the 3.8 percent NIIT on top of ordinary income tax. That extra layer widens the after-tax gap between the two options and is often left out of quick yield comparisons.
Running the Taxable Equivalent Yield
Tax-exempt funds almost always post a lower nominal yield than comparable taxable funds. The taxable equivalent yield tells you whether the tax savings close the gap:
TEY = Tax-Exempt Yield ÷ (1 − Your Marginal Tax Rate)
At a 37 percent federal bracket and a 2.5 percent tax-exempt yield, the TEY is 2.5% ÷ (1 − 0.37), or roughly 3.97 percent. A taxable money market fund would need to beat 3.97 percent to match the after-tax return. Add state tax to the denominator — a combined marginal rate of 45 percent, for instance — and the TEY rises to about 4.55 percent.
Run the same numbers at a 12 percent federal bracket and that 2.5 percent yield produces a TEY of only about 2.84 percent. A taxable fund yielding 3 percent easily beats it. At lower brackets, the tax savings do not compensate for the lower nominal yield.
Who These Funds Fit, and Who Should Skip Them
The category rewards high-income investors holding cash in a taxable brokerage account. The higher your combined federal and state marginal rate, the more the exemption pays. Residents of high-tax states, especially those using a single-state fund for the double exemption, get the biggest benefit.
Two groups should generally look elsewhere. Investors in the 10 or 12 percent federal brackets give up more in nominal yield than they save in tax; a taxable money market fund almost always leaves more cash behind. And anyone considering one of these funds inside an IRA, 401(k), or other tax-advantaged account is paying for a benefit they already have. Earnings inside those accounts are already tax-deferred or tax-free, so the lower yield delivers nothing in return.
Costs That Eat Into the Yield
Because the nominal yield is already modest, the expense ratio takes a proportionally larger bite than it would in a taxable fund. Ratios of 0.30 to 0.50 percent are common. Fund managers sometimes waive part of their fee to keep the net yield from falling to zero in low-rate environments, and when those waivers come off, the yield you see drops. Check the gross expense ratio, not just the net, so you know what happens if the waiver ends.
Stability, Insurance, and the 2023 Liquidity Rules
Money market fund shares are not insured or guaranteed by the FDIC or any government agency. That is the key line separating them from bank money market deposit accounts, which carry FDIC protection up to $250,000 per depositor. The rare event of a fund’s NAV falling below $1.00 is called breaking the buck, and while it has happened only a handful of times in industry history, it is a real risk.
The SEC overhauled money market fund rules in 2023. Funds can no longer impose temporary redemption gates that freeze your money during stress; under the prior rules, a board could block all redemptions for up to 10 business days, and that option is gone.7U.S. Securities and Exchange Commission. Money Market Fund Reforms In its place, institutional prime and institutional tax-exempt funds face a mandatory liquidity fee when daily net redemptions exceed 5 percent of net assets, defaulting to 1 percent if the cost of selling securities to meet those redemptions cannot be calculated; a de minimis exception applies if the calculated fee is under 0.01 percent. The SEC also raised minimum liquidity buffers to 25 percent of total assets in daily liquid assets and 50 percent in weekly liquid assets.8U.S. Securities and Exchange Commission. Money Market Fund Reforms – Form PF Reporting Requirements for Large Liquidity Fund Advisers
The mandatory liquidity fee applies to institutional share classes, not retail. If you invest through a retail tax-exempt money market fund, the fee provisions do not hit you directly. Stress in the municipal market can still affect your fund’s yield and holdings, so the higher liquidity buffers are the part of the 2023 reforms that matters most to individual shareholders.