The channel flow method with electrochemical detection (CFMED) has been used to investigate the photoelectrochemical reduction of O-2 at UV-illuminated TiO2 surfaces in both aerated and oxygenated solutions. This approach detects O-2 amperometrically at a working detector electrode strategically positioned downstream of a TiO2 surface, comprising a thin film of particulate material (Degussa P-25). By monitoring the transport-limited current as a function of flow rate with and without illumination, it has been possible to analyze the reaction kinetics in terms of various models proposed in the literature. The kinetics of the reduction process are reported, and the implications for semiconductor photocatalysis, are discussed.