The photocatalytic activity of anatase-structured Ti1-(5x/4)NbxO2-y-delta Ny (x = 0.25, y = 0.02; NbN-25) was examined for water oxidation under UV and visible light irradiation. The semiconductor was prepared by sol-gel processing followed by nitridation in flowing ammonia and exhibits an indirect optical gap of 2.2 eV. Ti1-(5x/4)NbxO2-y-delta Ny was loaded with RuO2 by an impregnation technique, and optimized conditions reveal that 1 wt % RuO2 generates 16 mu mol O-2 from water with concomitant IO3- reduction after 3 h of illumination under simulated solar radiation at a flux of 600 mW/cm(2) illumination, which corresponds to 6-sun AM1.5G illumination (compared to no detectible O-2 without the RuO2 cocatalyst). A series of cut-on filters shows that the catalyst-loaded semiconductor evolves O-2 for lambda <= 515 nm, and a gas-phase mass spectrometry isotope labeling experiment shows that irradiating an iodate solution in (H2O)-O-18 in the presence of 1 wt % RuO2 loaded on NbN-25 gives rise to catalytic water oxidation: both O-36(2) and O-34(2) are observed. It is unclear whether O-16 arises from IO3- or surface reconstruction on the photocatalyst, but ICP-AES analysis of the postirradiated solution shows no dissolved metal ions.