Nickel oxide thin films, known as optically active counter electrodes in electrochromic devices, were grown at room temperature under an oxygen pressure of 10(-1) mbar, using pulsed laser deposition (PLD). The influence of the film thickness, potential window, and extended cycling on the electrochromic properties was studied by means of complementary electrochemical, X-ray diffraction, and electron microscopy techniques. Independently of the substrate nature, transparent as-deposited films were crystallized with a [1 1 1] preferred orientation. The reversible color change from transparent to brownish is ascribed to a faradic process, involving the Ni3+/Ni2+ redox couple, the intensity of which is enhanced with decreasing film thickness. Finally, taking into account a first chemical reaction when NiO thin films are immersed in KOH, a mechanism based on hydroxide electrochemically active phases is proposed for the electrochromic properties of PLD NiO thin films. (C) 2004 The Electrochemical Society.