The surface of Monel‐400 alloy was exposed to high pH, aqueous corrosion at 285°C, under both oxidizing and reducing conditions. After exposures lasting from 0.3 to 335 hr the surfaces were examined by x‐ray photoelectron spectroscopy, scanning electron microscopy, and other techniques. Under reducing conditions, no corrosion film forms on the metal surface, but under oxidizing conditions, the first corrosion layer which forms at the solution interface is entirely . This hydroxide, which persists for long times at but which rapidly converts to at , is shown to result from a precipitation process. The composition of the corrosion film is determined by the relative solubilities of nickel and copper at the interface. A composition profile of the corrosion layer beneath the outermost film shows a gradual increase in cuprous ion concentration with increasing depth, probably due to solid‐state migration. This suggests that the aqueous corrosion of Monel‐400 involves both solid‐state oxide growth and an ionic dissolution and precipitation mechanism.