Films of Ni and Cr oxide were made by reactive DC magnetron sputtering. Ozone exposure, obtained by ultraviolet irradiation in the presence of oxygen, gradually induced coloration in the initially transparent films. Electrochemical measurements correlated the optical absorption with a charge deficiency in the film. Our results demonstrate a convenient technique for treating the counter electrode in a W-oxide-based electrochromic device prior to device assembly. (C) 2002 Elsevier Science B.V. All rights reserved.