The weights of ferric phosphate present in films formed by the passivation of iron in 0.1M solutions of disodium and trisodium phosphate were determined using a radioactive tracer, and the thickness of oxide was estimated by an electrochemical method. The weights of phosphate were found to decrease with a rise in pH value of the solution. Surface pretreatments, involving the removal of the original air‐formed oxide film by dipping in acid, resulted in higher phosphate contents than surface pretreatments involving abrasion, either with or without subsequent prolonged exposure to dry air.