Definition
Post-mix spending is the discipline of using coins after a CoinJoin without undoing the privacy you just gained. A mix improves fungibility by placing your output inside an anonymity set, but careless spending afterward can correlate that output back to your identity. The two most common mistakes are spending mixed outputs together with their leftover change, and consolidating many mixed UTXOs into one transaction, both of which give a chain analyst the links the mix was meant to break.
The change-output hazard
An equal-output CoinJoin typically leaves a non-standard change output that is trivially identifiable as yours. If you later spend that change in the same transaction as a mixed output, you re-associate them and effectively collapse your anonymity set. Best practice is to isolate change in a separate, non-private wallet and never co-spend it with mixed coins.
Consolidation and toxic change
UTXO consolidation can be fatal to privacy when done naively, because combining many post-mix outputs reveals common ownership. Sovereign users address this with coin control, spending mixed UTXOs individually where possible, labeling outputs, and avoiding merging coins from different anonymity sets. Research suggests an output's effective privacy stabilizes over time, but only if you do not actively re-link it. Thoughtful post-mix spending is what turns a one-time mix into durable fungibility.
See also CoinJoin coordinator.
In Simple Terms
Post-mix spending is the discipline of using coins after a CoinJoin without undoing the privacy you just gained. A mix improves fungibility by placing your…