A surface hydroperoxide intermediate has been detected upon oxidation of water at an Ir oxide nanocluster catalyst system under pulsed excitation of a [Ru(bpy)(3)](2+) visible light sensitizer by recording of the OO vibrational mode at 830 cm(-1). Rapid-scan FT-IR spectroscopy of colloidal H2O, D2O, and (D2O)-O-18 solutions in the attenuated total reflection mode allowed spectral assignment of IrOOH on the basis of an observed D shift of 30 cm(-1), and O-18 shifts of 24 cm(-1) ((OO)-O-16-O-18) and 46 cm(-1) ((OO)-O-18-O-18). The laser pulse response of the infrared band is consistent with the kinetic relevancy of the intermediate. This is the first observation of a surface intermediate of oxygen evolution at an Ir oxide multielectron catalyst.