The effects of deposition conditions, including substrate temperature and sputtering gas pressure; on the optical and electrochromic (EC) properties Of WO3 films prepared by RF reactive sputtering were investigated. The maximum optical bandgap energy of 3.15 eV was obtained on a room temperature substrate, and decreased with increasing substrate temperature. However, the maximum refractive index of about 2.5 was obtained at a sputtering gas pressure of 5 mTorr and a substrate temperature of 500 degreesC, and decreased with decreasing substrate temperature and with increasing pressure. The decrease in bandgap energy and refractive index are thought to have been caused by an increase in WO3 cluster size and a decrease in film density, respectively. Electrochromic (EC) response times of the WO3 films were measured in an electrolyte of 1 N H2SO4 aqueous solution. Fast EC responses were obtained for the WO3 films with wide bandgap energies and low refractive indices. The results indicate that the optical properties of WO3 films are useful indicators of EC response. (C) 2003 Elsevier B.V All rights reserved.