Physical vs Synthetic ETFs: Key Differences

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 financial institution, usually a large investment 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.

The short answer: A physical ETF owns the stocks in its index. A synthetic ETF does not. It gets the index return through a contract, usually a swap with one or more large financial institutions. Physical ETFs are simpler and more transparent, while synthetic ETFs carry a small counterparty risk but can track some indices more closely.

Table of contents

• Physical vs synthetic: side by side
• What is physical replication
• Full replication vs optimised replication
• What is synthetic replication
• Counterparty risk in synthetic ETFs
• Tracking difference vs tracking error
• Physical vs synthetic S&P 500 ETF: an example
• When synthetic makes sense
• How to tell which type a fund uses
• Common questions
• About Paasa

Physical vs synthetic: side by side

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 by a depositary, usually 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 financial institution (the counterparty), usually a large investment bank.

Under the swap, the counterparty agrees to pay the fund whatever the index returns. In exchange, the fund pays the counterparty an agreed amount, such as the return on its own assets plus 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.

Funded vs unfunded swaps

Synthetic ETFs use one of two swap structures, and the factsheet will usually tell you which:

• Unfunded swap: The fund uses your money to buy a basket of securities that it owns, often stocks that are not in the index. It then swaps the return of that basket for the return of the index. If the counterparty defaults, the fund still owns the basket. Most synthetic UCITS equity ETFs, including the Invesco S&P 500 UCITS ETF, use this structure.
• Funded swap: The fund pays your money to the counterparty, and the counterparty gives the fund collateral in return, held by a separate custodian. If the counterparty defaults, the fund relies on that collateral.

In both cases, your risk is limited to the difference between what the counterparty owes the fund and what the fund holds.

The tradeoff is counterparty risk.

Counterparty risk in synthetic ETFs

Counterparty risk is the risk that the counterparty 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 assets per counterparty if the counterparty is a bank, and 5% for any other 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.

Note: Under ESMA guidelines, the collateral a UCITS fund accepts must be liquid, high quality, and diversified, so it can be sold quickly if a counterparty defaults.

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.

Tracking difference vs tracking error

Factsheets show two numbers that sound alike but measure different things.

• Tracking difference: How much the fund's return differs from the index over a period. If the index returns 10% in a year and the fund returns 9.9%, the tracking difference is −0.1%. This is the real cost of holding the fund, including its fees.
• Tracking error: How much that difference moves around over time. A low tracking error means the fund follows the index consistently, even if it always trails it by the same amount.

When comparing two ETFs on the same index, tracking difference matters more. It tells you what you actually earned compared with the index.

Physical vs synthetic S&P 500 ETF: an example

Example: Suppose you want to invest in the S&P 500 through a UCITS ETF and are choosing between iShares Core S&P 500 (CSPX), which is physical, and Invesco S&P 500 (SPXS), which is synthetic. Both are domiciled in Ireland and track the same index. Here is how they compare, based on justETF data (returns in EUR):

The result: The synthetic ETF returned 1.08% more over four years

On a $10,000 investment, that is about $108 more. Only 0.02% a year comes from the lower TER. Most of the difference comes from dividend tax.

A physical Irish ETF pays 15% US tax on the dividends it receives. A synthetic ETF receives the index return through a swap instead, and under US Section 871(m) rules, swap payments on broad indices like the S&P 500 are not subject to this tax.

Note: Past returns do not guarantee future results, and this tax advantage depends on US rules staying the same. The synthetic ETF also carries counterparty risk that the physical one does not.

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% of assets per counterparty for banks, and 5% for others. 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.

Start investing in UCITS ETFs with Paasa