Affinity label is a molecule that is similar in structure to a particular substrate for a specific enzyme. It is considered to be a class of enzyme inactivators. These molecules covalently modify active site residues in order to elucidate the structure of the active site.

These types of compounds are highly reactive and usually alkylate nucleophilic amino acids present in the enzyme. They are often indiscriminant in their action, therefore their usefulness is limited as they are non-selective and therefore inherently toxic.