The role of defects on the electrochromic response time of sputter-deposited Ni oxide films grown by RF magnetron sputtering system was examined. In order to create an excess interstitial oxygens and voids, the sputter-deposited Ni oxides were grown at varying Ar/O-2 ambient ratios and RF power, respectively. The electrochromic response time was evaluated by an in situ transmittance measurement during a pulse potential cycling test. Although all of the sputter-deposited Ni oxide films had a similar, amorphous crystallographic structure, the excess interstitial oxygens and the voids affect the electrochromic response time, leading to a disturbance in proton intercalation/deintercalation during the coloring/bleaching processes. Moreover, the voids in the film played a more critical role on the electrochromic response time than an excess of interstitial oxygen. (C) 2003 Elsevier Science B.V. All rights reserved.