The ammonia monooxygenase (AMO) ofNitrosomonas europaeais a metalloenzyme that catalyses the oxidation of ammonia to hydroxylamine. We have identified histidine 191 of AmoA as the binding site for the oxidized mechanism-based inactivator acetylene. Binding of acetylene changed the molecular mass of His-191 from 155.15 to 197.2 Da (+42.05), providing evidence that acetylene was oxidized to ketene (CH₂CO; 42.04 Da) which binds specifically to His-191. It must be assumed that His-191 is part of the acetylene-activating site in AMO or at least directly neighbours this site.