Al was anodized in and electronmicroscopic studies made of breakdown and formation of pits beneath the film after various surface treatments or after additions of up to 0.05M Cl− to the acid and selection of those specimens which had remained in the film‐growth stage before stable pits grew. An electron‐opacity technique was developed for studying film growth. Even in the absence of Cl−, there was a continued incidence of breakdown and a substantial time‐dependent direct‐dissolution current. With Cl−, cell voltage tended to be lower. Film formation efficiency and geometrical structure were correlated directly with average cell voltage and not directly with Cl− concentration. Cl− was interpreted as having only an indirect effect on film growth by stabilizing concurrent pit growth, which reduced the film‐forming current. Possible modes of pitting beneath the film are discussed.