Cross-chain bridges: tax treatment in Spain

Bridges allow assets to be transferred between different blockchains. ETH from Ethereum to Arbitrum, BTC from Bitcoin to Ethereum (WBTC), SOL to Ethereum... These are everyday operations in DeFi but with tax implications that must be understood.

How does a bridge work?

General mechanics:

1. You deposit ETH into the Ethereum bridge contract.
2. The bridge emits wrapped ETH ("bridged ETH") on the destination chain.
3. When you want to come back, you burn the bridged ETH and recover the original ETH.

The key tax question: Is this a pass-through?

The criterion of ownership

The AEAT does not have specific doctrine on bridges, but the general criterion would be:

• If there is change of ownership (you transfer the asset to a third party) → transmission → taxable event.
• If there is no change of ownership (you are still the owner, you only change the form/network) → there is no transfer → there is no taxable event.

Most bridges operate without change of ownership: The contract holds your original ETH while you have the bridged ETH. When you return, you get back exactly what you deposited. It's analogous to... a wardrobe: you leave your clothes, they give you a ticket, you get the same clothes back.

Majority position of tax advisors: Bridges without change of real counterparty are not taxable transfers.

WBTC: the special case of wrapped tokens

Wrapped Bitcoin (WBTC) is BTC “wrapped” on the Ethereum blockchain as ERC-20. To get WBTC:

1. You send BTC to a centralized custodian (BitGo).
2. The custodian issues WBTC on Ethereum.

There is a centralized intermediary here and a more significant change in form. Some advisors consider that the "wrapping" of BTC in WBTC could be a transmission. The counterargument is that WBTC is backed 1:1 and you can always redeem BTC.

Conservative criteria for WBTC: Treat the wrap and unwrap as transmissions, recording the BTC price at both times.

Bridge hacks: the case of Ronin, Wormhole, Harmony

Bridge hacks have cost billions:

• Ronin Bridge (Axie Infinity): $615M stolen in March 2022.
• Wormhole: $320M in February 2022.
• Harmony Horizon Bridge: $100M in June 2022.

If your funds were in the bridge at the time of the hack:

• Asset loss. The asset is no longer in your control.
• Documentation: capture of the pre-hack balance, press articles about the hack, any communication from the team.
• When to declare: when the definitive loss is confirmed (with no prospect of recovery).
• If the protocol partially compensates you (like Wormhole did): The compensated amount is income. The uncompensated difference is the loss.

LayerZero, Stargate and other modern bridges

Modern bridges like Stargate (LayerZero) use shared liquidity pools:

• You deposit USDC in Ethereum → you receive USDC in Arbitrum from the Arbitrum pool.
• You are not receiving the same USDC that you deposited; you receive equivalent tokens from the pool.

Is this a broadcast? More likely yes, because you receive different tokens from the pool. The price of USDC is stable (€1≈€1), so the tax gain/loss is negligible. But it records the operations.

Bridge fees: deductible?

Bridge commissions (gas + protocol fee):

• If the bridge is part of a DeFi operation → deductible expense of that operation.
• If the bridge is to move own funds without generating income → less clear. In general, commissions paid in crypto are transmissions of a minimum amount of crypto.

Essential registration for bridges

For each bridge operation write down:

1. Date and time.
2. Chain of origin and destination.
3. Asset and quantity.
4. Transaction hash on the source chain.
5. Hash on the destination chain.
6. Bridge protocol used.

This is essential to prove that the funds appearing on a new chain are the same as those that came out of the other.

Updated: April 2026 | Fiscal year: 2025