Nickel oxide films were deposited by reactive RF magnetron sputtering. Optical and electrochemical characterization showed strong electrochrornic activity under certain deposition conditions. X-ray diffraction and infrared absorption measurements indicate that the films are polycrystalline NiO and that the bulk crystal structure does not change upon oxidation and reduction. Electronic conduction in the films and charge storage at the NiOlelectrolyte interface were studied by dc resistance and ac impedance measurements. The infrared measurements and x-ray photoelectron spectroscopy were used to characterize changes in surface composition. We interpret the experimental results as follows: electrochromic activity requires a porous, granular NiO film with excess oxygen at the grain surfaces. The film is reduced by the transfer of protons from water molecules to oxygen ions at the solid surface, and the film is oxidized by the transfer of protons from the solid surface to hydroxyl ions in solution.