In the chlorate process, it is known that a small addition of chromate to the electrolyte increases the current efficiency at the cathode by hindering the electrochemical reduction of intermediately formed hypochlorite. This paper presents results from in situ ellipsometric studies of the formation of the chromium(II1) hydroxide film in an attempt to further elucidate the consequences to the cathode surface. The results obtained are in good agreement with findings from conventional electrochemical and surface analytical techniques. The thickness of the chromium(II1) hydroxide layer obtained by cathodic reduction on evaporated gold electrodes is less than 4 nm after 2 h of film growth at -1 .O and -0.6 V rel. SCE. At -1 .O V, the initial film growth is rate limited by the nucleation process. With 0.15 mM chromate, the film attains a maximum thickness within 2 h of growth. When the chromate concentration is 15 mM, the rate of film growth rapidly decreases, but continuous growth is still in progress after 2 h of exposure. The film grows homogeneously and there are neither structural rearrangements within the film during build-up, nor changes in the mechanism of film growth with time. Studies of deposition and dissolution of the film indicate that the mechanistic path is the same in both directions.