Modification of tantalum nitride (Ta3N5), which has a band gap of 2.1 eV, with nanoparticulate iridium (Ir) and rutile titania (R-TiO2) achieved functionality as an O-2 evolution photocatalyst in a two-step water-splitting system with an IO3-/I- shuttle redox mediator under visible light (lambda > 420 nm) in combination with a Pt/ZrO2/TaON H-2 evolution photocatalyst. The loaded Ir nanoparticles acted as active sites to reduce IO3- to I-, while the R-TiO2 modifier suppressed the adsorption of I- on Ta3N5, allowing Ta3N5 to evolve O-2 in the two-step water-splitting system.