Definition
"Tainted coins" is the informal label for bitcoin that blockchain-analysis firms or exchanges have flagged because their transaction history links them, directly or several hops back, to activity considered illicit, such as theft, ransom payments, darknet markets, or sanctioned addresses. Taint analysis is the practice of tracing those flows across the public ledger and assigning a risk score to specific coins or addresses. The term carries no formal legal definition; it is a product of commercial surveillance heuristics, not a property of the protocol.
How coins get flagged
Because every Bitcoin transaction is permanently recorded on a public ledger, analysis companies cluster addresses, follow value through transactions, and brand outputs that pass through flagged points. Custodial exchanges, under pressure to demonstrate AML compliance, screen incoming deposits against these scores. A user can receive flagged coins entirely unknowingly and later find a custodial deposit delayed, frozen, or questioned.
The fungibility problem
Fungibility means every unit of a money is interchangeable with every other unit of equal value. Tainting directly attacks this property: if some bitcoin are treated as "dirtier" than others, identical coins no longer trade at par, undermining a core requirement of sound money. Bitcoiners therefore treat taint scoring as a threat to fungibility rather than a neutral fact, and privacy techniques exist specifically to break the analytical assumptions that make tainting possible.
This entry is educational and not legal advice. For related concepts, see AML (Anti-Money Laundering) and KYC (Know Your Customer).
In Simple Terms
“Tainted coins” is the informal label for bitcoin that blockchain-analysis firms or exchanges have flagged because their transaction history links them, directly or several hops…