Photo by Tötös Ádám on Unsplash
Ask a few people whether an S&P 500 index fund and an S&P 500 ETF from the same company are basically interchangeable, and most will say yes. They hold the same 500 stocks in the same weights, so the reasoning goes, and the rest is just packaging. The holdings part of that claim is true. The “just packaging” part is where it breaks, and the difference shows up on your tax return, not in your portfolio’s performance.
Where the Claim Is Right
If a fund company offers both a mutual fund index fund and an ETF tracking the same index, they typically do own the identical basket of underlying securities. Vanguard’s S&P 500 index fund and its S&P 500 ETF (VOO) track the same 500 companies at the same weights. For a long-term, buy-and-hold investor building a position through regular contributions, the exposure you get is the same either way.
Where It Breaks: How Each One Is Built
The difference is structural, not cosmetic. According to Fidelity, when a traditional mutual fund index fund faces redemptions, the manager has to sell securities to raise cash, and that sale can create a capital gain distributed to every remaining shareholder, even ones sitting on an unrealized loss overall. An ETF instead uses “creation units,” swapping baskets of securities in kind rather than selling for cash, so an individual investor usually isn’t exposed to capital gains triggered by other shareholders’ trading.
That single mechanical difference is why the two structures behave so differently at tax time, even when the underlying index is identical.
Photo by Nicholas Cappello on Unsplash
The Number That Shows It
According to State Street’s own 2025 data, just 4% of passive ETFs distributed a capital gain in 2025, versus 41% of passive index mutual funds. The gap is even wider on the active side: American Century reports that about 8% of active equity ETFs paid out capital gains, against nearly 76% of active equity mutual funds.
| Fund type | Share that distributed a capital gain (2025) |
|---|---|
| Passive index ETFs | 4% |
| Passive index mutual funds | 41% |
| Active equity ETFs | ~8% |
| Active equity mutual funds | ~76% |
Two funds can hold the exact same 500 stocks and still send very different tax bills, because the difference was never about what they hold. It’s about how shares get created and redeemed.
Where This Actually Matters
This entire distinction is about taxable brokerage accounts. Inside a 401(k), traditional IRA, or Roth IRA, capital gains distributions don’t trigger a current tax bill either way, so the structural difference is close to irrelevant there. If every dollar you’re comparing sits in a retirement account, this is a distinction without a difference for you. It only bites in a regular taxable account, where a distribution is a tax event whether or not you sold anything yourself.
The Exceptions Worth Knowing
The ETF tax advantage isn’t universal. Fidelity names several structures where it doesn’t fully apply:
- Leveraged and inverse ETFs get “60/40” IRS treatment (60% long-term, 40% short-term gains) regardless of how long you hold them.
- Emerging-market ETFs often can’t use in-kind delivery and have to sell for cash instead, narrowing the gap.
- Commodity ETPs face similar 60/40 treatment because of the derivatives they use.
A plain S&P 500 or total-market index ETF gets the full benefit described above. A leveraged 2x fund or a commodity ETP does not.
Photo by Kelly Sikkema on Unsplash
❓ FAQ: Frequently Asked Questions
Do index funds and ETFs tracking the same index hold the same stocks?
Usually yes, when offered by the same company against the same benchmark. Vanguard’s S&P 500 mutual fund and its S&P 500 ETF (VOO) hold the identical 500 companies at the same weights.
Which is more tax-efficient, an index fund or an ETF?
ETFs, structurally. In 2025, State Street reports just 4% of passive ETFs distributed a capital gain versus 41% of passive index mutual funds, due to the in-kind creation and redemption process ETFs use.
Does this tax difference matter in a 401(k) or IRA?
No, not meaningfully. Capital gains distributions inside tax-advantaged retirement accounts don’t create a current tax bill, so the structural advantage of ETFs is mostly irrelevant there.
Are all ETFs equally tax-efficient?
No. Leveraged, inverse, emerging-market, and commodity ETFs each lose some or all of the standard advantage, either through mandatory IRS 60/40 gain treatment or because they can’t use in-kind redemptions.
Figures in this piece come from Fidelity, State Street (SSGA), and American Century’s own published tax-efficiency data, and from 2026 capital gains bracket reporting following the IRS’s Revenue Procedure 2025-32 inflation adjustments. Bracket thresholds and NIIT limits change with inflation adjustments; verify the current year’s numbers before relying on them for a tax decision, and this is not personalized tax advice.