Rust formed by corrosion on the inner surfaces of oil tanks may react with hydrogen sulfide gas in the vapor above crude oil. This reaction produces mixtures of iron sulfides which are pyrophoric, and they cause explosions when they are exposed to air during the unloading of the tankers. This paper shows that it is possible, by exposure to an atmosphere of nitrogen containing 2 or 4 vol % oxygen, to deactivate these sulfides with mackinawite, FeS, pyrite, FeS2, and greigite, Fe3S4. Mackinawite undergoes slow oxidation to give goethite, FeO(OH), which has a lower reactivity. Pyrite and greigite are pyrophoric, present in the mixture in a finely divided state and produce sparks on exposure to air. During deactivation they do not oxidize but transform to a framboidal morphology which has a lower surface area and is less reactive.