When you buy an ETF, you are buying a wrapper. The question is what is inside.
Two ETFs can track the same index, charge a similar TER, and look identical on the surface. But one holds the actual stocks. The other holds a contract with a bank that promises to deliver those returns. These are physically and synthetically replicated ETFs, and understanding the difference matters for knowing what you actually own.
Table of contents
- What is physical replication
- Full replication vs optimised replication
- What is synthetic replication
- Physical vs synthetic: side by side
- Counterparty risk in synthetic ETFs
- When synthetic makes sense
- How to tell which type a fund uses
- Common questions
- About Paasa
What is physical replication
A physically replicated ETF buys and holds the actual securities in its index.
If you invest in a physical S&P 500 ETF, the fund owns shares in all 500 companies in the S&P 500, weighted to match the index. When the index rebalances, the fund buys and sells to stay aligned.
When you hold units in a physically replicated ETF, you are a beneficial owner of a proportional share of those underlying securities. The assets are held in custody by a bank, separately from the fund manager's own balance sheet. If the fund manager goes out of business, the assets are still there, segregated, and returned to investors.
Full replication vs optimised replication
Within physical replication, there are two approaches.
Full replication means the fund holds every single security in the index at the correct weight. This is practical for indices with a small number of large-cap liquid stocks, like the S&P 500 or the MSCI World.
Optimised (or sampling) replication means the fund holds a representative subset of the index rather than every constituent. This is used when full replication is expensive or impractical, for example in an index that includes small illiquid companies in frontier markets. The fund selects a basket that closely mirrors the index's behaviour without holding every stock.
Both are considered physical replication. The fund still holds real securities. The only difference is whether it holds all of them or a representative sample.
What is synthetic replication
A synthetically replicated ETF does not hold the securities in its index. Instead, it enters into a swap agreement with a bank (the counterparty).
Under the swap, the bank agrees to pay the fund whatever the index returns. In exchange, the fund pays the bank a fee. The fund may hold a basket of other assets as collateral, but these assets may have no relationship to the index it is tracking.
From a return perspective, a synthetic ETF can track its index very precisely, often with lower tracking difference than a physical fund. The swap agreement transfers the return efficiently, without the fund needing to buy and sell securities every time the index rebalances.
The tradeoff is counterparty risk.
Physical vs synthetic: side by side
| Physical | Synthetic | |
|---|---|---|
| Holds underlying securities | Yes | No |
| Tracks index via | Owning assets | Swap agreement |
| Counterparty risk | Low (securities lending only) | Yes (capped at 10% under UCITS) |
| Tracking difference | Slightly higher in some cases | Often lower |
| Transparency | High | Lower |
| Common uses | Equity indices, broad market | Commodities, leveraged products, hard-to-access markets |
| US ETF examples | VOO, IVV, SPY, QQQ | UPRO, SQQQ (leveraged/inverse) |
| UCITS ETF examples | CSPX, VWRA, EQQQ, IWDA | WisdomTree commodity ETFs, some Asia ETFs |
Counterparty risk in synthetic ETFs
Counterparty risk is the risk that the bank on the other side of the swap defaults before paying you what it owes.
UCITS regulations cap this exposure at 10% of the fund's NAV per counterparty. In practice, most synthetic UCITS ETFs reset the swap daily and hold collateral to keep the exposure well below that limit. Large providers like iShares or Xtrackers also work with multiple counterparties to diversify the risk further.
But the structural exposure exists in a way it does not for physical funds. If the swap counterparty defaults, the fund must liquidate the collateral and use it to compensate investors. Whether that collateral is sufficient depends on how well it has been managed and how the default scenario unfolds.
This risk is real but small for well-structured UCITS funds. It is larger in poorly structured products or those with concentrated counterparty exposure.
When synthetic makes sense
Synthetic replication is not inherently inferior to physical. There are situations where it is the better structure.
Commodities and hard-to-access assets: You cannot physically hold oil, wheat, or carbon credits in a fund. Synthetic replication via futures or swaps is how commodity ETFs work.
US-listed stocks in non-US markets: Some exposures are easier or cheaper to replicate synthetically due to market access constraints or high transaction costs.
Leveraged and inverse ETFs: These products use derivatives by nature. The synthetic structure is integral to how they work.
Lower tracking difference: For some indices, a well-run synthetic ETF consistently delivers tighter tracking than a physical one, because it avoids the friction of buying and selling securities on rebalance.
For standard equity index ETFs covering liquid markets, physical replication is the default and the most transparent option. For everything else, the right structure depends on what the index contains.
How to tell which type a fund uses
The replication method is a required disclosure for UCITS funds. You will find it in:
- The fund's prospectus or base prospectus
- The KIID (Key Investor Information Document)
- The fund factsheet, usually under "Fund Details" or "Investment Policy"
- JustETF, which lists replication method on each fund's page under "Replication"
The terminology varies slightly by provider. Look for "physical", "full replication", "optimised", "sampling", "synthetic", or "swap-based". Some providers describe the method in one word; others explain it in a sentence.
Note: Some ETF names include terms like "Physical" or "Acc" (accumulating) in the fund name. "Physical" in the name typically confirms physical replication. "Acc" refers to the dividend treatment (accumulating vs distributing) and says nothing about replication method.
Common questions
Does replication method affect how dividends are handled?
No. Dividend treatment (accumulating vs distributing) is separate from replication method. A physical ETF can be accumulating or distributing. So can a synthetic ETF.
Are all UCITS ETFs physical?
No. UCITS regulations permit synthetic replication with counterparty exposure capped at 10% per counterparty. Most broad equity index UCITS ETFs are physical, but synthetic UCITS ETFs exist and are common in commodity, leveraged, and niche exposures.
Does synthetic replication affect how the ETF is taxed in India?
Not directly. Indian tax rules look at whether the fund is equity-oriented or not for the purpose of applying capital gains rates, not at the internal replication method. Consult a CA for your specific situation.
About Paasa
Paasa is a global investing platform built for Indian investors. We provide direct access to UCITS ETFs listed on the London Stock Exchange, Euronext, and Xetra, alongside equities across 10+ global exchanges including the US, UK, Germany, Switzerland, Japan, Hong Kong, and Singapore.
For Indian investors buying ETFs through Paasa, the India-specific layer is handled end to end:
- Schedule FA reporting: We generate the exact foreign asset disclosure reports your CA needs for your Indian tax return, mapped to the Indian financial year.
- LRS and FEMA support: We handle remittance coordination and ensure your overseas investments stay within the RBI's Liberalised Remittance Scheme limits.
- Tax filing and advisory: Access to expert tax advice on capital gains, dividend income, and cost basis calculations for foreign holdings.
