A combination system consisting of a H2 production photocatalyst, Pt/SrTiO3:Rh, and an O2 production photo-catalyst, BiVO4 or WO3, decomposed water into H 2 and O2 under visible light irradiation in the presence of an Fe3+/Fe2+ redox couple as an electron mediator. O2 evolution on the BiVO4 photocatalyst was inhibited by Fe2+ ions, because of the oxidation of Fe2+ instead of water. In contrast, H2 evolution on the Pt/SrTiO3:Rh photocatalyst was enhanced when Fe3+ ions co-existed. It is due to the back-reactions between H2 and O2 to form water, and the reduction of Fe3+ by H2, which easily proceeded on the bare Pt cocatalyst surface, being efficiently suppressed by adsorption of [Fe(SO4)(H2O)5]+ and/or [Fe(OH)(H2O)5]2+ on the Pt surface. Overall water splitting was achieved with the suppression of the back-reactions even using a Pt cocatalyst. Thus, it clears that iron ions contributed to the present Z-scheme systems not only as an electron mediator but also as an inhibitor of the back-reactions.