A buffered tris(2,2＇-bipyridyl)ruthenium complex-colloidal IrO2 system was studied as a photocatalyst for the production of O-2 from water. Phosphate buffer, which has historically been used to control the pH in this system, accelerates the decomposition of the photosensitizer and inhibits O-2 evolution, whereas sodium hexafluorosilicate (Na2SiF6)-base solutions are ideal buffers for the reaction. Na2SiF6-containing buffers poise the solution under visible light irradiation at ca. pH 5, preventing the pH drop that accompanies oxidation of water in unbuffered solutions. Decomposition of the photosensitizer is not kinetically competitive with oxygen evolution in these buffers. In particular, the Na2SiF6-NaHCO3 buffer greatly improves the turnover number of the photosensitizer, relative to previously used phosphate buffers, without any decrease in activity. Photocatalytic reactions studied under various conditions suggest that adsorbed carbonate or bicarbonate on the surface of the colloidal IrO2 particles contributes to the increased turnover number of the photosensitizer.