Hydrogen peroxide was produced as a solar fuel from water and dioxygen using solar energy by combination of a water oxidation catalyst and a photocatalyst for two-electron reduction of O2 in acidic aqueous solutions. Photocatalytic production of H2O2 occurred under photoirradiation of [RuII(Me2phen)3] 2+ (Me2phen = 4,7-dimethyl-1,10-phenanthroline) used as a photocatalyst with visible light in the presence of Ir(OH)3 acting as a water oxidation catalyst in an O2-saturated H2SO 4 aqueous solution. Photoinduced electron transfer from the excited state of [RuII(Me2phen)3]2+ to O2 results in the formation of [RuIII(Me 2phen)3]3+ and a superoxide radical anion (O2-) which is protonated to produce H2O 2via disproportionation of HO2 in competition with back electron transfer (BET) from O2- to [Ru III(Me2phen)3]3+. [Ru III(Me2phen)3]3+ oxidises water with the aid of catalysis of Ir(OH)3 to produce O2. The photocatalytic reactivity of H2O2 production was improved by replacing Ir(OH)3 nanoparticles by [CoIII(Cp*) (bpy)(H2O)]2+ in the presence of Sc(NO3) 3 in water. The optimised quantum yield of the photocatalytic H 2O2 production at λ = 450 nm was determined using a ferrioxalate actinometer to be 37%. The value of conversion efficiency from solar energy to chemical energy was also determined to be 0.25%.