The passive film on iron in acid media is investigated by means of a rapid change of electrolyte composition, with special attention to anion content, and by redox kinetics studies with a rotating disk under steady state and nonsteady state (electrohydrodynamic impedance). These techniques, associated with classical electrochemical impedance, allow one to establish the existence at the film‐electrolyte interface of a loosely structured film limiting the rate of mass transfer in the case of ferrocyanide oxidation. Dynamic exchange of anions and most of the electrolyte composition effects take place in this layer. An attempt is made to relate the behavior of the passive film on iron in acid media to basic properties of iron oxides and oxyhydroxides in their colloidal state.