Electrochemical studies, as well as microscope observations, on an Fe‐24% Cr alloy in have shown that at least two distinct, potential‐dependent films are formed. The primary film, which is responsible for the initial passivation, is stable only within a few millivolts of the primary activation potential. A secondary film, which forms at more positive potentials, grows to a thickness greater than 10Aå and, with increasing potential and time, becomes very stable and resistant to reduction. The primary passivation process is reversible. The “thickness” of the primary film at the primary activation potential corresponds to less than the equivalent of one oxygen atom per surface metal atom. From this “thickness” and from the pH‐dependence of the primary activation potential (81 mv/pH), it is suggested that the primary passivation process forms a film containing chromium and oxygen ions and possibly also some constituent of the electrolyte solution.