Physical vs. Synthetic ETFs — What Experienced Investors Need to Know
The replication method rarely makes or breaks an ETF investment — but it measurably drives the tracking difference. On US indices in particular, synthetic ETFs have systematically delivered better results for years, while the much-cited counterparty risk is more tightly regulated than many assume. Time for a sober assessment.
Counterparty risk: real, but regulated
A swap ETF holds a substitute basket and exchanges its return for the index return via a swap. The risk lies in the swap counterparty defaulting — but UCITS rules cap the uncollateralised swap exposure at 10% of fund assets, and in practice daily resets and collateral keep it well below that.
The mirror image: physical ETFs are not risk-free either. Many lend out parts of their portfolio for a fee (securities lending) — also a collateralised counterparty risk that partly finances the fund’s cost gap. The categorical "physical safe, synthetic risky" framing does not survive scrutiny.
Concentration risks, ETF overlap and look-through analysis – free with MoneyPeak.
The withholding-tax edge of synthetic US ETFs
The tangible reason for synthetic replication: physical UCITS ETFs (Irish domicile) pay 15% withholding tax on US dividends. Swap-based ETFs on qualified indices such as the S&P 500 can replicate the gross return — the withholding tax effectively disappears.
Quantified: at a dividend yield of around 1.5% on the S&P 500, the 15% withholding tax equals roughly 0.2 percentage points per year. On a €100,000 portfolio that is about €200 annually — a five-figure sum over 20 years with compounding. This is exactly why synthetic S&P 500 ETFs have topped tracking-difference rankings for years. For the bigger product picture, see the big global ETF comparison.
| Criterion | Physical | Synthetic (swap) |
|---|---|---|
| Index replication | direct holdings (full or sampling) | swap on a substitute basket |
| Counterparty risk | securities lending (collateralised) | swap, max 10% uncollateralised (UCITS) |
| US withholding tax | 15% on dividends (Ireland) | effectively none on qualified indices |
| Typical TD on US indices | close to TER | often beats the index |
| German partial exemption | 30% | 30% (with a 51% equity basket) |
German tax practice and decision logic
Since the 2018 German investment tax reform, swap ETFs are on equal tax footing: the 30% partial exemption applies as long as the fund continuously holds at least a 51% equity quota — which mainstream swap ETFs ensure via their substitute basket. The advance lump-sum tax and the 26.375% flat tax including solidarity surcharge apply identically.
- US-heavy indices (S&P 500, MSCI USA): synthetic has a structural return advantage.
- MSCI World and European indices: the edge is smaller; product quality matters more than replication — see for instance the Xtrackers MSCI World.
- What ultimately counts is the realised tracking difference over several years, not the factsheet TER.
Frequently asked questions
How big is the risk of losing money to a counterparty default in a swap ETF?
UCITS caps the uncollateralised swap exposure at 10% of fund assets; collateral and regular resets keep actual exposure usually far lower. A total loss from a counterparty default is practically ruled out.
Does the German 30% partial exemption also apply to synthetic ETFs?
Yes, provided the ETF continuously maintains at least a 51% equity quota. Mainstream swap ETFs meet this through their physical substitute basket.
Why do some synthetic S&P 500 ETFs beat their index?
They replicate the gross return while the benchmark uses the net return after 15% US withholding tax. At a dividend yield of around 1.5%, that is roughly a 0.2 percentage point advantage per year.
Concentration risks, ETF overlap and look-through analysis – free with MoneyPeak.