The effects of molybdenum [Mo] doping on the electrochromic behavior of spray pyrolised tungsten trioxide [WO3] thin films have been studied. It has been observed that the color-bleaching kinetics, coloration efficiency, and stability of electrochromic WO3 films are closely related to molybdenum doping concentration, apart from their microstructure and crystallinity. While a nominal 6.0 at.% molybdenum doping produces best electrochromic response in WO3 films, the electrochemical stability is highest when the nominal concentration of molybdenum is about 2.0 at.%. The improved electrochromic behavior of the Mo doped WO3 films has been explained from the improved H+ ion diffusion coefficient in the films during coloration and decoloration process. (C) 2010 Elsevier Ltd. All rights reserved.