Incorporation of a small amount of Ca2+ ions into a polymeric cobalt cyanide complex to form Cax[CoII(H2O)2]1.5-x[CoIII(CN)6] resulted in a significant enhancement of activity for photocatalytic water oxidation in a buffer solution (pH 7.0) containing [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) as a photocatalyst and Na2S2O8 as an electron acceptor to achieve a quantum efficiency of 200%.