Modification of silica-supported vanadium oxide (VS) photocatalysts with alkali metal ions enhanced the photocatalytic activity of the vanadium(V) mononuclear complex for oxidative decomposition of gaseous organic compounds into carbon dioxide. Action spectrum analysis revealed that the onset wavelength for cesium-ion-modified VS was 420 nm, which was ca. 20 nm longer than that with a titania photocatalyst, P25.