Rutile IrO2 is known as being among the best electrocatalysts for water oxidation. Here we report on the unexpected photocatalytic water oxidation activity of 1.98 nm +/- 0.11 nm succinic acid-stabilized IrO2 nanocrystals. From aqueous persulfate and silver nitrate solution the nonsensitized particles evolve oxygen with initial rates up to 0.96 mu mol min(-1) and with a quantum efficiency of at least 0.19% (measured at 530 nm). The catalytic process is driven by visible excitations from the Ir-d(t(2g)) to the Ir-d(e(g)) band (1.5-2.75 eV) and by ultraviolet excitations from the O-p band to the Ir-d(e(g)) (>3.0 eV) band. The formation of the photogenerated charge carriers can be directly observed with surface photovoltage spectroscopy. The results shed new light on the role of IrO2 in dye- and semiconductor-sensitized water splitting systems.