Two share classes of one fund
iShares Core S&P 500 UCITS ETF exists in two forms that track the very same index, with the same manager and the same ongoing charge (about 0.07%):
- an accumulating class — CSPX, ISIN
IE00B5BMR087— which keeps the dividends inside the fund and reinvests them; - a distributing class — IUSA, ISIN
IE0031442068— which pays the dividends out to you in cash.
Everything an investor usually compares — index, cost, provider, replication — is identical. What is not identical is how a Belgian resident is taxed on them, and the difference runs in two opposite directions at once.
The two taxes pull opposite ways
For a Belgian investor these two classes sit on different sides of two separate taxes:
| CSPX (accumulating) | IUSA (distributing) | |
|---|---|---|
| TOB, per buy and per sell | 1.32% | 0.12% |
| Belgian 30% dividend tax | none — nothing is paid out | applies to each distribution |
The accumulating class pays no distribution, so the Belgian 30% dividend withholding never reaches the investor. But an accumulating class registered in Belgium carries the top 1.32% TOB on every purchase and every sale. The distributing class is the mirror image: a low 0.12% TOB, but its dividends are exposed to the 30%.
So "which is cheaper" is not a matter of taste. It is a small piece of arithmetic that turns on how long you hold and how large the position is.
The arithmetic, on €10,000
Take a €10,000 purchase, and the S&P 500's dividend yield of roughly 1.15%.
TOB, paid once at purchase:
- CSPX: 1.32% = €132
- IUSA: 0.12% = €12
The accumulating class costs €120 more up front — and it will cost more again on the way out, because the 1.32% versus 0.12% gap applies to the sale too, on whatever the position is then worth.
Dividend tax, every year:
- IUSA distributes about 1.15% → roughly €115 a year, on which the Belgian 30% is about €34.50 a year.
- CSPX distributes nothing, so this line is zero.
Set the one-off €120 against the recurring €34.50, and the distributing class's dividend tax overtakes the accumulating class's extra purchase TOB after about three to four years — before even counting the second TOB hit on the eventual sale.
What that points to
For the investor an S&P 500 tracker is usually bought for — a long-horizon, rarely-traded, buy-and-hold position — the arithmetic points to the accumulating class. Over ten or twenty years the avoided dividend tax adds up to far more than the one-off TOB difference, and the accumulating class also spares you the annual work of declaring and reclaiming dividends.
The distributing class is the better tax fit in the opposite situation:
- a short holding period, where the higher TOB never has years of dividend tax to be weighed against;
- frequent trading or rebalancing, where the 1.32% is paid over and over;
- a small portfolio whose total dividends stay under the reclaimable annual tranche (often quoted around €833), because then much of IUSA's Belgian 30% can be recovered through the tax return, shrinking the very cost the accumulating class exists to avoid;
- a need for the dividends as cash income.
This is a description of how the two tax regimes compare, not a recommendation to buy either fund. The right choice also depends on things that have nothing to do with tax.
Two things not to misread
The 1.32% is broker-sensitive. As set out in Why Bolero and DEGIRO tax the same ETF differently, whether an accumulating class is charged 1.32% or 0.12% depends on how a broker reads its Belgian registration — so the accumulating class's TOB disadvantage is not identical everywhere. Check the rate your own broker actually applies.
Accumulating does not avoid every dividend tax. Inside the fund, the underlying US dividends are subject to US withholding before they are reinvested — and that happens in both classes, accumulating and distributing alike. What the accumulating class avoids is only the Belgian 30% at the investor level, not the tax that occurs inside the fund.
Always verify your figures
Belfolio computes and presents these amounts for information only. This is not tax advice, and it is not investment advice. The yield, the TOB rate your broker applies and the reclaimable dividend tranche all change — verify them with the FPS Finance or your accountant before acting on them.
Published 7 Aug 2026