Photocatalytic degradation of aqueous cyanide was studied. Tungstophosphoric acid(TPA)-modified TiO₂ such as SG-2 (home-made), P25 (commercial), and SG-4 (home-made) showed better activity than the other pure titania such as anatase TiO₂ and rutile TiO₂ both of which were commercial ones. The activity arose from both surface hydroxyl groups and adsorbed water molecules formed on the catalysts during the reaction. The surface hydroxyl groups were effectibely suppressed by fluoride-exchange treatment causing the decrease of the activity of the catalysts. The drop in activity was unambiguously obvious for the pure titania in the presence of iodide as a hydroxyl radical scavenger. For TPA-modified TiO₂, meanwhile, the diminution was such a small extent that it suggested that direct oxidation by photogenerated holes rather than indirect oxidation via hydroxyl radicals was the main reaction pathway.