An inhibitory substance characterized by its propensity to attach very strongly, by a true @[email protected] (e.g. covalent) to the surface atoms or ions constituting the catalytically active sites. Poisons act in minute quantities. Typical poisons are S, As, etc. In most cases, activity and/or @[email protected] cannot be recovered without a drastic change in operating conditions (most often a regeneration). @[email protected], if at all, takes place very slowly and/or only partially.