Guide

Money market funds explained

A retiree sells $180,000 of stock and the proceeds sit in a brokerage sweep account earning 0.01% while she waits to rebalance. Three clicks away, the same cash could sit in a government money market fund paying a seven-day yield above 4%. The trade-off is not obvious: MMFs are mutual funds, not bank deposits; they aim for a stable $1.00 share price but can “break the buck” in stress; and tax treatment differs from high-yield savings or T-bills. Money market funds (MMFs) pool investor cash to buy very short-term, high-quality debt — Treasury bills, repurchase agreements, commercial paper, certificates of deposit — and pass interest income back to shareholders. They are the default parking vehicle inside brokerage accounts and a core tool for emergency reserves held at investment firms. This guide covers fund types, NAV mechanics, sweep accounts, yield math, comparisons to bank cash and direct Treasuries, tax nuances, a Harbor Treasury cash sleeve worked example, a product decision table, common pitfalls, and a retail checklist.

What a money market fund is

An MMF is a regulated mutual fund that invests only in short-maturity, liquid instruments. Under U.S. Securities and Exchange Commission Rule 2a-7, funds must maintain a weighted average maturity (WAM) of 60 days or less and a weighted average life (WAL) of 120 days or less, hold diversified high-quality paper, and limit illiquid assets. Investors buy and redeem at the fund’s net asset value (NAV), which for most retail MMFs is managed to stay at $1.00 per share through amortized-cost accounting and sponsor support.

Unlike a bank savings account, an MMF is not FDIC-insured. Protection comes from portfolio quality, liquidity rules, and — in government funds — holdings limited to U.S. government securities and repurchase agreements collateralized by them. You earn return through daily accrual of interest, distributed as dividends (often monthly) rather than a posted APY on a deposit statement.

Government vs prime vs municipal

Government MMFs invest at least 99.5% of assets in cash, U.S. government securities, and repos backed by government collateral. They carry the lowest credit risk and became the default after 2016 reforms ended stable NAV for institutional prime funds. Tickers like SPAXX, VMFXX, and SWVXX are household names in retail brokerage sweeps.

Prime MMFs may hold commercial paper, certificates of deposit, and corporate short-term debt in addition to government paper. Yields are typically higher because credit spreads add income, but prime funds experienced severe stress in 2008 and March 2020 when institutional investors fled. Retail prime funds still exist but are less common in default sweep programs; know what you own before accepting extra yield.

Municipal (tax-exempt) MMFs hold short-term state and local debt. Dividends are generally exempt from federal income tax and sometimes state tax if you buy a fund focused on your state of residence. Yields look lower on paper; the after-tax return can beat taxable government funds for investors in high marginal brackets.

Stable NAV and breaking the buck

Retail MMFs quote a constant $1.00 share price so investors think in dollar terms: deposit $50,000, see $50,000, plus dividends. Behind the scenes, the market value of holdings fluctuates. When losses exceed a fund’s buffer, the NAV can fall below $1.00 — breaking the buck.

The Reserve Primary Fund broke the buck in September 2008 after Lehman Brothers commercial paper defaulted, freezing redemptions and accelerating the broader money-market panic. Reforms since then imposed liquidity fees, redemption gates for institutional funds, and stricter asset quality. In March 2020, prime institutional funds again faced heavy outflows; several sponsors merged or closed funds rather than let NAVs collapse, but the episode reminded investors that MMFs are not risk-free cash.

Government funds have never broken the buck in the U.S. retail context because their collateral is Treasuries and government repos. That history — not a legal guarantee — is why conservative cash investors default to government MMFs over prime.

Brokerage sweep accounts

When you sell stock or receive dividends, uninvested cash often lands in a sweep program that automatically transfers balances into a partner MMF (or bank deposit program) at the end of each business day. Sweep rates track the fund’s seven-day yield minus a small platform spread; some brokers let you choose among government, prime, or municipal sweep options.

Check three settings on every new account: (1) which fund is the default sweep, (2) whether idle cash below a threshold stays uninvested earning nothing, and (3) settlement timing — stock sale proceeds may be “available to trade” before they sweep, leaving a one-day gap at zero yield. Moving cash manually into a preferred MMF ticker before a long weekend can capture extra days of accrual.

Bank sweep programs (FDIC-insured deposits at partner banks) compete with MMF sweeps at some brokers. Compare insured deposit limits against MMF yield and your comfort with fund credit risk; neither is automatically superior.

Reading yields: seven-day SEC yield

Fund fact sheets quote a seven-day SEC yield: the annualized income the fund earned over the past seven days, assuming dividends are reinvested, net of expenses. It is the standard apples-to-apples comparison across MMFs but differs from bank APY in two ways.

First, MMF yields move daily with short rates; a fund paying 4.8% today may pay 4.2% next month if the Federal Reserve cuts. Second, SEC yield is gross of your personal taxes unless you hold a municipal fund. When comparing to a certificate of deposit, remember CDs lock a rate for a term while MMF income floats with policy rates — advantageous when rates rise, painful when they fall.

Expense ratios matter at the margin. A government fund with a 0.09% fee versus 0.42% on an identical portfolio delivers meaningfully more net yield over years on a six-figure balance. Institutional share classes (often $1M minimums) offer lower fees; retail investors should pick the lowest-fee government option their platform offers.

MMF vs HYSA vs T-bills vs CDs

All four are cash-parking tools with different plumbing:

MMF — mutual fund shares, no FDIC, T+1 redemption at brokerage, yield floats, best inside investment accounts already open.
HYSA — bank deposit, FDIC/NCUA insured to limits, variable APY, best for standalone emergency cash and bill-pay linkage.
T-bills — direct U.S. government obligation, state-tax exempt on interest, require TreasuryDirect or broker purchase, maturity dates to manage.
CDs — locked rate and term, early withdrawal penalties, FDIC insured, beat MMFs when you are certain of the holding period and rates are peaking.

A practical split: keep six months of living expenses in an HYSA for instant bank transfer and insurance clarity; park investable cash and sale proceeds in a government MMF at the brokerage where you trade; ladder T-bills or CDs only when you have a dated liability (tuition, tax payment) and want rate certainty.

Tax treatment

Taxable MMF dividends are generally ordinary income, reported on Form 1099-DIV, not qualified dividend rates. High earners in states with income tax should compare after-tax yield on a government MMF against a state-specific municipal MMF or direct Treasuries (exempt from state tax).

Some government fund dividends include a percentage of income exempt from state tax because underlying assets are federal obligations; fund websites publish annual percentages for state return worksheets. Municipal MMFs may generate alternative minimum tax (AMT) exposure from private activity bonds — check the fund’s AMT percentage if you are near AMT thresholds.

Unlike bank interest, MMF dividends in a taxable account do not compound visibly on your 1099 until year-end unless you reinvest; plan quarterly estimated tax payments if yields are large relative to withholding.

Worked example: Harbor Treasury cash sleeve

Harbor Endowment manages a $2.4M portfolio with a 5% strategic cash sleeve ($120,000) for opportunistic rebalancing and six-week operating reserves. Policy requires government-only MMFs or T-bills — no prime exposure.

In March the team holds $85,000 post-trade proceeds at the custodian’s default bank sweep (0.35% APY, FDIC insured) and $35,000 already in VMFXX (4.62% seven-day yield). They move the $85,000 into VMFXX, raising blended cash yield from roughly 1.5% to 4.5% on the full sleeve — about $3,600 additional annual income versus leaving sale proceeds in bank sweep.

Tax note: Harbor is a taxable trust in a 37% federal bracket. Municipal MMFs do not clear the hurdle after adjusting for lower gross yields. They skip T-bill ladders for this sleeve because rebalance triggers are unpredictable and MMF liquidity is same-day. The investment policy statement documents MMF tickers, prohibits prime funds, and requires quarterly verification that sweep settings match the policy — a step many retail investors skip until they discover months of 0.01% default cash.

Product decision table

Your situation	Preferred vehicle	Watch for
Brokerage idle cash between trades	Government MMF sweep	Default sweep fund; sub-$1 balances not swept
Emergency fund, standalone	HYSA at online bank	FDIC limit per bank; transfer speed
High federal + state tax bracket	State municipal MMF or T-bills	AMT % on muni funds; state tax exemption rules
Known expense in 6 months	CD or T-bill ladder	Early withdrawal penalty; reinvestment at lower rates
Institutional cash > $250k at one bank	MMF or multi-bank sweep	FDIC cap per depositor per bank
Yield chasing in calm markets	Prime MMF (optional)	Credit spreads widening; redemption gates (institutional)
Retirement account cash	Government MMF in IRA	Tax shelter makes muni funds inefficient inside IRA

Common pitfalls

Assuming sweep cash is optimized — default programs often use low-yield bank deposits or legacy prime funds.
Treating MMFs like FDIC insurance — credit and liquidity risk exist, especially in prime funds.
Comparing SEC yield to CD APY without term context — locked CDs win when rates fall; MMFs win when rates rise.
Ignoring expense ratios — 0.30% difference is $300/year per $100,000.
Holding municipal MMFs in IRAs — you forfeit tax exemption inside tax-deferred accounts with no benefit.
Leaving purchase proceeds unsettled — cash may not sweep until settlement (T+1 for stocks).
Chasing prime yield before Fed easing — spread compression can erase the premium quickly.
Account sprawl — five small MMF positions across brokers complicate tax reporting for minimal yield gain.

Retail checklist

Log into each brokerage and note the current sweep fund ticker and seven-day yield.
Confirm sweep is government-only unless you explicitly accept prime credit risk.
Compare sweep yield to the platform’s manual-purchase government MMF options and expense ratios.
Pair MMF cash with an HYSA emergency bucket so insurance and liquidity cover different needs.
Estimate after-tax yield: taxable government MMF vs muni MMF vs T-bills for your bracket and state.
Set calendar reminders after Fed meetings to re-check whether CD or bill ladders beat floating MMF yields.
Verify 1099-DIV classification each January; report ordinary dividend income correctly.
For balances above FDIC limits at banks, confirm whether MMF or multi-bank sweep is the better fit.
Document tickers in your investment policy or personal finance spreadsheet.
Revisit quarterly — broker defaults change without email notice.

Key takeaways

Money market funds are short-term mutual funds, not insured deposits; government MMFs are the conservative default.
Sweep accounts automate cash into MMFs but defaults are often suboptimal until you change them.
Seven-day SEC yield is the comparison metric; it floats with policy rates unlike locked CD APYs.
Tax treatment favors municipal MMFs or Treasuries for high brackets; muni funds waste tax benefits inside IRAs.
Pair MMFs with HYSAs — brokerage cash for investing workflow, bank cash for insured emergency reserves.