The several experiments performed with r.f. sputtered NiOx films in aqueous KOH electrolytes indicate that during the cathodic electrochromic reaction hydrogen is incorporated in the oxide film up to a concentration of about 10% of the Ni atoms. This reaction is associated with a compressive stress increase of 2 × 109 N/m2 in the thin film. The electrochromic reaction reversibly transforms H-rich, bleached films (transmittance up to 90%) into H-poor dark colored films (transmittance down to 20%) and vice versa. Experiments performed at pH 11 indicate that the coloration reaction mechanism may exhibit as rate-determining step the mass transfer of OH- to the film - electrolyte interface.