Tax-Exempt Municipal Money Market Funds

Which of these are you?

• You hold cash in a taxable brokerage account and your marginal federal rate is 24% or higher. Price a tax-exempt fund against your best taxable one with the tax-equivalent yield below. This is the case the product exists for.
• You live in a state with no income tax, or your marginal federal rate is 12% or lower. The exemption is worth little. A taxable government or bank product usually wins on after-tax yield.
• You live in a high-tax state. Look for a fund built from your own state's bonds. A national fund exempts the federal tax only, and your state may tax the dividends.
• The cash is in an IRA, a 401(k), or an HSA. Skip this chapter. You would be paying, in lower yield, for an exemption you cannot use.

What the fund is and what it holds

A tax-exempt money market fund is a mutual fund that buys short-dated debt issued by states, cities, school districts, hospitals and other public borrowers, and passes the interest to you as exempt-interest dividends. Rule 2a-7 defines a tax exempt fund as one that distributes income exempt from regular federal income tax (17 CFR 270.2a-7(a)(23)).

It is a money market fund, so the same rulebook that governs a taxable fund applies. Every security has a remaining maturity of 397 calendar days or less, the portfolio's weighted average maturity is capped at 60 days, and its weighted average life at 120 days (Rule 2a-7(d)(1)). The fund may not buy a security other than a weekly liquid asset if that would leave it below 50 percent of total assets in weekly liquid assets. A 25 percent daily liquid asset minimum applies to money funds generally, but the rule says that provision does not apply to tax exempt funds (Rule 2a-7(d)(4)(ii) and (iii)). The 2023 amendments raised the daily and weekly minimums to 25 percent and 50 percent; tax-exempt funds have only the weekly one (Rule 2a-7(d)(4)(ii)).

None of that makes it a bank product. The FDIC states plainly that mutual funds, including money market funds, are not covered by deposit insurance. The chapter on the map of cash choices sets a tax-exempt fund beside the other options.

How the exemption works, and where it stops

The federal exemption is real and it is statutory. A regulated investment company can pay exempt-interest dividends if, at the close of each quarter of its taxable year, at least 50 percent of its total assets are obligations described in 26 U.S.C. 103(a). Shareholders then treat those dividends as interest excludable from gross income (26 U.S.C. 852(b)(5)). A fund built to pay exempt-interest dividends is designed to clear that test, so for a money fund it is background, not a live constraint.

Three limits sit around that exemption.

You still report it. The IRS tells filers to report all taxable and tax-exempt interest on the federal return (Tax Topic 403), and the fund reports exempt-interest dividends to you in box 12 of Form 1099-DIV. The reporting is informational, but it matters because other calculations read that line.

It is not the same as tax free under the alternative minimum tax. Interest on specified private activity bonds is a preference item under 26 U.S.C. 57(a)(5)(A). Private activity bonds are municipal bonds where the proceeds mainly benefit a private user, such as an airport, a port or a housing program. Some categories are carved out by statute, including qualified 501(c)(3) bonds, certain housing bonds, and bonds issued from 2009 through 2010. The fund reports the private-activity share of your dividends separately in box 13 of Form 1099-DIV, and it flows to line 2g of Form 6251. For tax year 2026, the alternative minimum tax exemption is $140,200 for joint filers and phases out starting at $1,000,000 of alternative minimum taxable income (IRS 2026 inflation adjustments, Rev. Proc. 2025-32). These amounts change every year, so check the current Form 6251 instructions. A household needs substantial alternative minimum taxable income before the tax applies. If you owe it, the box 13 figure is part of the calculation, and a fund advertised as tax-exempt is less exempt for you than for someone who does not owe it.

Your state has its own rules. The federal exclusion does not automatically carry into your state return. The two states below show the two patterns.

Other states differ, so read your own revenue department's instruction for exempt-interest dividends before you assume anything. The state chapter covers how the state rate enters the comparison.

The tax-equivalent yield, with every step

A tax-equivalent yield is the taxable yield you would need to match the exempt yield after tax. Formula: TEY = exempt yield / (1 - t). Here, exempt yield is the fund's tax-exempt yield, and t is the tax rate that applies to the taxable alternative, written as a decimal. In this chapter, t is the federal marginal rate, or the federal rate plus your state rate when the exempt fund is also exempt from your state. Adding the two assumes you receive no federal benefit from deducting the state tax, so it is a simplification. All yields below are hypothetical.

Take an exempt yield of 3.00%.

Now add a state. With a 24% federal rate and a 6% state rate on a fund that is exempt from both, t is 0.30 and the tax-equivalent yield is 3.00 / 0.70, which is 4.286%. With a 32% federal rate and the same 6%, t is 0.38 and the figure is 3.00 / 0.62, or 4.839%.

Run your own rate through the calculator below. It is a screening tool: it takes the tax rate you enter and does not know your bracket, your state or your alternative minimum tax position.

A dollar example on $100,000

Suppose you hold $100,000 of cash in a taxable account. A tax-exempt fund yields 3.00% and a taxable fund yields 4.00%. Both figures are hypothetical. The exempt fund pays $100,000 x 3.00% = $3,000 a year. The taxable fund pays $100,000 x 4.00% = $4,000 before tax.

After tax, the taxable fund pays $4,000 x (1 - t). The exempt fund pays $3,000 if it is exempt from every tax you owe, or $3,000 x (1 - state rate) if the state taxes it.

Three things stand out. First, the same fund loses in the first row and wins in the second, and the only change is the tax rate. Second, the state layer is where the biggest swings come from: the in-state fund earns $200 more than the taxable fund at a 24% federal rate, while the out-of-state fund earns only $20 more, because the state taxes it anyway. Third, the break-even is easy to state. The exempt yield needed to tie a 4.00% taxable yield is 4.00% x (1 - t): 3.04% at 24% federal, 2.80% at a combined 30%, 2.72% at 32% federal, and 2.48% at a combined 38%.

Who actually benefits

The benefit needs three conditions at once: the money sits in a taxable account, your marginal rate is high, and you do not need daily bank-style features such as checks or FDIC coverage. Miss any one and the answer flips. In an IRA the exemption is wasted. At a low marginal rate the tax-equivalent yield barely rises above the exempt yield. If you need insurance, only a deposit account provides it.

Compare the exempt fund's net yield with the taxable fund's net yield, both after expenses. Two funds with the same holdings and different expense ratios are two different offers, and a tax-equivalent calculation multiplies whatever mistake is in the input.

Use the exempt fund for money you would otherwise leave in a taxable brokerage sweep. That is the Tier 2 reserve or Tier 3 goal money in the framework, held in a taxable account, not the Tier 1 cash you need tomorrow.

What you take on for the exemption

A tax-exempt fund is not a deposit, and the 2023 rule changes shape how it behaves under stress. The SEC's 2023 amendments removed the ability to suspend redemptions, raised the minimum liquidity buffers, and required institutional prime and institutional tax-exempt funds to impose a mandatory liquidity fee when daily net redemptions exceed 5 percent of net assets, unless liquidity costs are de minimis. Non-government funds must also be able to impose a discretionary fee when the board decides it is in the fund's interest, and Rule 2a-7(c)(2) caps that discretionary fee at 2 percent.

Which fund you own matters. Under Rule 2a-7(c)(1), a retail money market fund, defined as one with policies limiting all beneficial owners to natural persons, and government funds may hold a stable share price. Institutional funds that are not government funds must price their shares to at least four decimal places, which means a floating share price. A tax-exempt fund you buy as an individual may be either retail or institutional, so check the prospectus for which one you hold. In a stress episode an institutional fund's price and fees can move. A retail fund keeps a stable price under the rule, though the SEC also lets retail funds float their price or reduce shares outstanding if rates turn negative.

Here is the arithmetic on a hypothetical redemption. If you sell $50,000 of shares while a fund imposes a fee at the 2 percent cap, the fee is $50,000 x 0.02 = $1,000. The cap is a ceiling, not a typical charge, and a mandatory fee is a good-faith estimate of liquidity cost rather than a fixed percentage.

Add two risks that are specific to municipal paper. Credit quality varies by issuer, so a fund concentrated in a few borrowers or one state is more exposed to that borrower or state. Rule 2a-7 also defines a single state fund, one that seeks to maximize income exempt from a particular state's taxes, and that concentration is the price of the in-state exemption.

Variable rate demand notes in one paragraph

Many tax-exempt money funds hold variable rate demand notes, which are municipal bonds whose rate resets periodically, often daily or weekly, and which the holder can put back at par at each reset. A remarketing agent resells tendered bonds and sets the new rate, and a backstop liquidity facility such as a letter of credit or a standby bond purchase agreement pays holders if the agent cannot find a buyer (MSRB EMMA). Rule 2a-7 defines a demand feature as a right to sell at approximately amortized cost plus accrued interest (17 CFR 270.2a-7(a)(9)). For you, the point is that a fund's liquidity in these holdings depends on the remarketing agent and the bank behind the facility, not only on the municipality that issued the bonds. You do not buy these notes yourself as a saver. You hold them through the fund.

A decision routine

1. Confirm the account is taxable. If it is a retirement account, stop.
2. Write down your marginal federal rate and your state's rate. Add them only for a fund exempt from both, and note that this ignores any federal benefit from deducting state tax.
3. Ask the fund company what share of last year's dividends was exempt from your state, and what share appeared in box 13 of Form 1099-DIV.
4. Compute the tax-equivalent yield and compare it with the best taxable fund you would actually use, after expenses.
5. Check whether the fund is retail or institutional, and read what its prospectus says about liquidity fees.
6. Recheck once a year. Tax rates, the alternative minimum tax exemption and fund holdings all change.