The photoinduced surface wettability conversion reactions of ZnO and TiO2 thin films were investigated by means of water contact angle measurement and X-ray photoelectron spectroscopy. Before ultraviolet (UV) illumination, ZnO and TiO2 films exhibited water contact angles of similar to 109 and similar to 54 degrees, respectively. UV illumination turned both surfaces to highly hydrophilic with water contact angles smaller than 10 degrees, Storage in the dark reconverted the highly hydrophilic films to their original states. Reversible surface wettability conversion reactions were achieved by alternate UV illumination and storage in the dark on both the films. The similar behaviors of wettability conversion observed on ZnO and TiO2 surfaces suggest that they follow a similar conversion mechanism. Preferential adsorption of water molecules on the photogenerated surface defective sites is ascribed to the formation of highly hydrophilic ZnO and TiO2 surfaces. Achievement of highly hydrophilic ZnO and TiO2 surfaces by high-temperature annealing and Ar+ sputtering provided supporting evidence for the explanation that surface defective sites play crucial roles in causing the surface wettability conversion reactions.