The formation and oxidation of nickel hydroxide films under a variety of experimental conditions are studied using in situ ellipsometric measurements. Two forms of the hydroxide, which we assume to be the α and β forms reported by other workers, are found. The α‐ and β‐hydroxide layers are both transparent with real refractive indices of 1.52 and 1.46, respectively, at 6328Aå. Upon oxidation the two forms of hydroxide convert to different oxides, both of which absorb light. The process involved in the conversion of the β‐hydroxide appears to be similar to the process postulated for the charging of the nickel battery electrode.