The response of an RF sputtered NiOx thin film to step potential and its electrochromic mechanism have been investigated. It was found that the NiOx films manifested fine electrochromic properties. Bleaching and coloration did not cause the change of the film's structure. The NiOx films are non-stoichoimetric with Ni vacancies. It is suggested that the bleaching and coloration reaction of the NiOx films in a KOH solution occurs along the grain boundaries. The physical absorption of colored NiOx films is caused by the intraband absorbance of the t(2g) level of the Ni3+ ion. (C) 2002 Elsevier Science B.V. All rights reserved.