The electrochromic, photochromic and photoelectrochemical properties of thin semiconductor WO3 films, prepared from quantum-size colloids, have been examined. These films have been found to exhibit reversible electrochromic and photochromic behavior (blue coloration). The trapped electrons in the particulate films are thought to be the major species responsible for the blue coloration. The photoelectrochemical measurements have shown a very low conversion efficiency of WO3 particulate films. This is most probably due to the efficient trapping of electrons by the lattice defects in the film.