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The short answer#
Most investors who avoid synthetic ETFs do so for one reason. The fund does not own what it tracks, and a bank stands between them and the index. The worry is legitimate, and the second half of this guide measures how large it is.
The other half of the trade gets less attention. A physical S&P 500 fund domiciled in Ireland pays US tax on every dividend its shares receive. A synthetic fund tracking the same index at the same ongoing charge generally does not. Over a year the gap can be as large as the fee, and because it is deducted before any return is reported, it never appears in a cost comparison.
So the decision has two numbers in it. One is a tax saving you can calculate from the index yield. The other is a credit exposure to a bank, which the UCITS rules cap and require to be collateralised. This guide works through both and then sets out when each should win.
How physical replication works#
A physically replicating fund buys the shares in its index and holds them through its depositary, in accounts opened in the fund's name. An S&P 500 fund of this kind owns stakes in the five hundred companies at roughly their index weights. What it reports and what it owns are the same thing.
When those companies pay a dividend, the United States withholds tax before the money leaves the country. A treaty sets the rate, and for an Irish fund the treaty in question is the one between the United States and Ireland. The IRS's own summary table gives the general rate on dividends under it as 15%, citing Article 10(2).
That 15% is taken inside the fund before the dividend arrives. It is no part of the TER, and you cannot reclaim it, because it was charged to the fund and not to you. It surfaces in one place only, as part of the gap between the fund's return and its index.
How a fund's return is taxed further down the line has its own guide, layer by layer. This one stays with the first layer, because it is the only one the replication method changes.
How synthetic replication works#
A synthetic fund takes investors' money and buys a basket of securities that need have nothing to do with the index. It then enters a total return swap with one or more banks. The fund pays the bank whatever its basket returns, and the bank pays the fund the return of the index. The fund follows the S&P 500 because of the contract, while what it owns can be European shares, Japanese shares or bonds.
A second version exists, in which the fund passes its cash to the bank and takes collateral back. Both are permitted under UCITS, and the prospectus says which one a fund uses.
Why the swap escapes the withholding#
US law does reach derivatives. Section 871(m) of the Internal Revenue Code treats a payment under a swap that tracks a US dividend as a dividend equivalent, and withholds on it as though it were the dividend itself. So the treaty is not what helps the synthetic fund, and neither is 871(m) in general.
What helps is one paragraph of the regulations under that section. A qualified index is treated as a single security that is not an underlying security, so a swap paying the return of one does not reference the American shares inside it, and no dividend equivalent arises. The conditions are spelled out. The index needs at least 25 components, none above 15% of the weight, and its five largest together at no more than 40%. It must be rebalanced on published criteria, and futures or options on it must trade on a qualifying exchange. A broad index like the S&P 500 is the case those tests were drawn around, and another test in the same paragraph uses the S&P 500's own dividend yield as its yardstick.
Two consequences follow, and a prospectus will not print either in bold. The exemption holds for as long as the index keeps passing those tests, and the 40% ceiling on the five largest names is the one to watch on an index whose biggest holdings keep getting heavier. The rules themselves are also unsettled. In Notice 2026-61, published in September 2026, the IRS extended the phase-in of the 871(m) regulations through 2028 and said that it and the Treasury continue to evaluate them. A tax advantage resting on a regulation under review is a reason to keep checking, and no reason to panic.
What the gap comes to#
The saving is the product of two numbers, the index dividend yield and the 15% a physical fund loses on it.
Take an illustrative yield of 1,2%. It is an assumption chosen for round arithmetic, and the current figure is on the index provider's factsheet. A physical fund loses 15% of 1,2%, which is 0,18% of its assets a year. A swap that pays the full gross dividend loses none of it.
The bank does not provide this for nothing. The swap carries a spread, and part of the saving can go to the counterparty and never reach the fund. You see what is left in the fund's tracking difference, the gap between fund and index over a full year with every cost included. That number settles which fund is cheaper to hold. The TER cannot, since two funds at the same TER can sit nearly a fifth of a per cent apart on this alone.
One trap when comparing them. A factsheet measures the fund against a particular version of the index, and the net and gross versions assume different withholding. Check that both funds are measured against the same version before you compare their tracking differences.
1,0%
- Lost by a physical fund, per year
0,15%
- On a €100.000 holding
150 €
1,2%
- Lost by a physical fund, per year
0,18%
- On a €100.000 holding
180 €
1,5%
- Lost by a physical fund, per year
0,225%
- On a €100.000 holding
225 €
Counterparty and collateral risk#
Counterparty risk is the chance that the bank on the other side of the swap fails while it owes the fund money. It is why many investors stop reading at the word synthetic, and the UCITS framework answers it in three places.
The 10% cap#
Article 52 of the UCITS Directive limits the risk exposure to a counterparty in an OTC derivative to 10% of the fund's assets where the counterparty is a credit institution, and to 5% in other cases.
The exposure is what the bank owes the fund at that moment, net of collateral. It is a different figure from the size of the swap. The fund's whole return runs through the contract, while the amount it stands to lose to one bank's failure is held under the limit. As the index rises the amount owed grows, and the fund stays inside the cap by having the swap settled or by taking collateral against it. The prospectus says how and how often.
ESMA's guidelines add one more condition. The basket the fund holds must itself meet the UCITS diversification limits, so the fund's own assets are spread across issuers like any other UCITS, even though they are not what it tracks.
What the collateral has to be#
Collateral that counts against counterparty exposure must meet ESMA's criteria at all times. The ones a private investor can read for are these.
It must be highly liquid and valued at least daily, of high credit quality, and issued by an entity independent of the counterparty whose performance is not expected to move closely with it. It must be diversified, with no more than 20% of the fund's net asset value exposed to one issuer. The exception is government debt, where a fund may be fully collateralised across at least six issues with no single issue above 30%. Under a title transfer the depositary holds it, and the fund must be able to enforce it at any time without the counterparty's approval. Non-cash collateral may not be sold, reinvested or pledged, and the fund needs a documented haircut policy for each kind of asset it accepts.
The fund's prospectus has to name its swap counterparties and describe the risk of their default, and its annual report has to list them along with the type and amount of collateral received. Every one of these facts can be checked for a specific fund before you buy it.
If the bank fails#
In the structure where the fund buys its own basket, the basket belongs to the fund and sits with the depositary, out of the bank's reach. If the bank defaults, the fund keeps the basket, and what the bank owed it under the swap becomes a claim against an insolvent bank, less any collateral the fund holds against it. That claim is the part the 10% limit caps.
What the rules leave open is what happens next. The fund now holds a basket that is not the S&P 500. It can sign a new swap with another counterparty, which is one reason funds use several, or it can sell the basket, and the prospectus says which routes it has. Until one is taken, tracking can slip by more than a year of withholding saving. Weigh the risk in those terms, bounded in size and uncertain in timing.
When each one wins#
The matrix ranks no fund and names none. It sets out which way the mechanism points in each situation, and the last column gives the reason, so you can test it against your own facts.
A US-only index such as the S&P 500, held for the long term
- Leans towards
Synthetic, if you accept the bank exposure
- Why
The whole fund is exposed to US dividends, so the saving is the full yield × 15%, less the swap spread.
A global index
- Leans towards
Either. Physical is the simpler default
- Why
The swap avoids US withholding only on the US part of the index, so the saving scales with the US weight. The choice among physical global funds has its own guide.
Emerging markets
- Leans towards
Physical, unless the tracking record says otherwise
- Why
The 871(m) exemption is a US rule. Whether a swap saves anything on other countries' dividends depends on its terms, so compare tracking differences and assume nothing.
You want no exposure to a bank at all
- Leans towards
Physical
- Why
No swap, no counterparty. You still carry the depositary arrangements every UCITS has.
You want the fund's holdings to be the index
- Leans towards
Physical
- Why
A synthetic fund's holdings list shows its basket, which can bear no resemblance to what you are exposed to.
You already hold a physical S&P 500 fund
- Leans towards
Usually keep it
- Why
Switching means selling, and the tax on the gain can exceed many years of a 0,1–0,2% saving.
Four things to read before buying either#
All four are in documents the fund is required to publish.
The replication method, on the factsheet. It decides which half of this guide applies.
The swap counterparties and the default risk, in the prospectus.
The collateral received and its type, in the latest annual report.
The tracking difference over several full years, with both funds measured against the same version of the index.
