Hydrated ZrO2 thin films were prepared by reactive sputtering in H2O atmosphere at different substrate temperatures. The crystal structure, chemical bonding state, film density, refractive index, surface morphology and ion conductivity of the films were investigated. The intensity of the absorbance peaks due to hydrogen bonded OH groups increased and the film density decreased with decreasing substrate temperature. Correspondingly, the ion conductivity increased with decreasing substrate temperature and a maximum ion conductivity of 6 x 10(-6) S/m was obtained for the film deposited at the lowest substrate temperature of -30 degrees C. We considered that the addition of H2O in the reactive gas and substrate cooling were very effective in increasing the H2O content and decreasing film density, which resulted in the high proton conductivity of the films. (C) 2011 Elsevier B.V. All rights reserved.