AbstractAn “Inhibitor” is something added to a corrosive liquid to render it non‐corrosive; “passivity” is a change in the metal remaining after it has emerged from the “inhibiting” bath, enabling it to resist liquids which normally would corrode it. The object of the paper is to ask whether passivity is due to (1) a three‐dimensional film (2) a two‐dimensional film (chemisorbed material) or (3) a change within the metal or alloy.The evidence supports the idea that a three‐dimensional film is usually necessary. For instance, iron which has been rendered passive in chromic acid, or has remained in water containing chromate long enough to make it safe to reduce the chromate concentration without risk of rusting, does carry a three‐dimensional film. Doubtless the “decision” between the formation of a protective film or of a soluble salt will be reached at the “chemi‐sorption” stage; but a chemi‐sorbed film does not seem to be capable of protecting the metal from further action by the film‐forming agent or by any liquid which would normally corrode it.There seems to be no clearly established case where intrinsic corrosion‐resistance is obtained by alloying, independent of film‐formation; but the possibility deserves further investigation.