Nonstoichiometric nickel oxide films were made by reactive dc magnetron sputtering. Subsequent electrochemical treatment in KOH established electrochromism. The material was studied by combined mechanical and optical measurements during electrochemical cycling, spectral infrared reflectance, XPS, and X-ray diffraction. Mechanical stress data showed that electrochromic bleaching was associated with proton insertion. P-polarized infrared reflectance showed OH stretching vibrations representative of 'free' OH for the bleached state and OH in the presence of hydrogen bonds for the coloured state. XPS data for O1s core level electrons indicated one binding state in the bleached material and two binding states in the coloured material. Heat-treated samples had significantly decreased electrochromism.