Tax-exempt money market funds are mutual funds that hold short-term municipal debt so the interest they distribute to you is generally free from federal income tax. They aim to keep a steady $1.00 share price, offer daily liquidity, and pay a tax-favored yield that can beat a taxable money market fund on an after-tax basis if you sit in a high enough bracket. Whether they actually beat one for you depends on your marginal rate, your state, and where you’re holding the cash.
What Makes the Income Tax Free
The tax break traces back to the securities inside the fund. These funds buy debt issued by state and local governments and their agencies to finance public projects like schools, highways, water systems, and airports. Federal law has long excluded the interest on that debt from gross income. The current exclusion lives in Section 103 of the Internal Revenue Code, which provides that gross income does not include interest on any state or local bond, 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 passes it along to you, the income keeps its tax-free character. The fund reports these payments as exempt-interest dividends on Form 1099-DIV, Box 12, and you don’t owe federal income tax on them.2Internal Revenue Service. Instructions for Form 1099-DIV Any capital gains the fund realizes from selling securities are a separate matter and remain taxable.
One thing the tax break does not do is turn the fund into a bank account. Money market fund shares are not insured or guaranteed by the FDIC or any government agency, which sets them apart from bank money market deposit accounts protected up to $250,000 per depositor.
Federal Only, or State Too
The federal exclusion is universal. It doesn’t matter which state issued the bond or where you live. If the interest qualifies under Section 103, it’s free from federal income tax.1Office of the Law Revision Counsel. 26 USC 103 – Interest on State and Local Bonds State income tax is different. Most states only exempt interest on bonds issued within their own borders. Live in Massachusetts, and the interest your fund earns on a Texas-issued bond will generally be taxed by Massachusetts.
That distinction produces two flavors of fund:
- National funds. These hold municipal debt from issuers across the country. You reliably avoid federal income tax, but most of the income will still be subject to your state’s income tax.
- Single-state funds. These concentrate holdings in one specific state. If you live in that state, you can pick up a double exemption, meaning no federal tax and no state or local tax on the interest.
A high-income New York City resident might choose a New York-specific fund to dodge federal, New York State, and New York City income taxes on the same dollar of interest. The trade-off is concentration risk. A single-state fund depends on the creditworthiness of issuers within one state, so you give up the diversification a national portfolio offers.
When the AMT Bites
Not all municipal interest escapes every federal tax. Interest on certain private activity bonds, meaning municipal bonds whose proceeds primarily benefit a private entity rather than the general public, counts as a tax preference item under the alternative minimum tax.3Municipal Securities Rulemaking Board. Tax Treatment4Internal Revenue Service. TEB Phase II – Lesson 4 General Rules for Private Activity Bonds That interest is still excluded from the regular income tax, but it gets added back when you calculate AMT liability.
Whether that matters depends on your overall tax picture. The AMT has its own exemption amount, and for 2026 it sits significantly higher than it did before 2018, so many taxpayers clear it entirely. If your income, deductions, and preference items push you into AMT territory, though, private activity bond interest can partially erode the tax savings you thought you were getting.
Fund companies know this and offer AMT-free tax-exempt money market funds that screen out private activity bonds altogether. If you suspect you might owe AMT in a given year, an AMT-free fund takes the guesswork off the table.
The Net Investment Income Tax Break
High earners subject to the 3.8 percent Net Investment Income Tax get a genuine bonus here. The NIIT 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 it doesn’t count toward your MAGI for this calculation.5Internal Revenue Service. Topic No. 559, Net Investment Income Tax
That’s a real extra layer of savings that yield comparisons often miss. A taxable money market fund paying the same nominal rate would trigger the 3.8 percent NIIT on top of ordinary income tax, widening the gap between the two options.
Doing the Math: Taxable Equivalent Yield
Tax-exempt funds almost always pay a lower nominal yield than comparable taxable funds. The question is whether the tax savings close the gap. The standard tool for answering it is the taxable equivalent yield:
TEY = Tax-Exempt Yield ÷ (1 − Your Marginal Tax Rate)
Say you’re in the 37 percent federal bracket and a tax-exempt fund yields 2.5 percent. Your TEY works out to 2.5% ÷ (1 − 0.37), or roughly 3.97 percent. A taxable money market fund would need to beat 3.97 percent just to match the after-tax return of the tax-exempt option. Fold state tax into the denominator, say a combined federal and state marginal rate of 45 percent, and the TEY climbs to about 4.55 percent.
The math looks very different at lower brackets. For someone in the 12 percent bracket, the same 2.5 percent tax-exempt yield translates to a TEY of only about 2.84 percent. Any taxable fund yielding 3 percent or more beats it easily. The tax savings simply aren’t large enough at lower brackets to justify accepting a lower nominal yield.
Who These Funds Are For, and Who Should Skip Them
The sweet spot is a high-income investor who needs a liquid place to hold cash outside of a retirement account. The higher your combined federal and state marginal rate, the more the exemption is worth. Residents of high-tax states, where combined marginal rates can approach or exceed 50 percent, get the biggest lift, especially through a single-state fund that captures the double exemption.
Two groups should generally stay away. First, investors in the 10 or 12 percent federal brackets. The nominal yield sacrifice outweighs the modest tax benefit, and a taxable money market fund will almost always leave more cash in your pocket. Second, anyone thinking about holding a tax-exempt fund inside an IRA, 401(k), or other tax-advantaged account. Earnings in those accounts are already tax-deferred or tax-free, so the fund’s lower yield buys you nothing. You’d be giving up return for a tax break you’re not using.
What to Watch For
Because these funds already pay lower nominal yields than taxable peers, the expense ratio takes a proportionally bigger bite. Ratios of 0.30 to 0.50 percent are common, and some managers voluntarily waive part of their fees to keep the net yield from dropping to zero or turning negative in low-rate environments. When those waivers come off, your yield drops with them. Look at both the gross expense ratio, which is what it actually costs to run the fund, and the net expense ratio, which is what you’re paying today, because the waiver can be pulled at any time.
One SEC rule change often causes confusion: a 2023 reform introduced a mandatory liquidity fee that kicks in when daily net redemptions exceed 5 percent of a fund’s net assets.6U.S. Securities and Exchange Commission. Money Market Fund Reforms – Form PF Reporting Requirements for Large Liquidity Fund AdvisersMoney Market Fund Reforms