The photocatalytic cleavage of water over Pt-RuO2/TiO2 catalysts under near UV illumination is investigated in a semicontinuous flow photoreactor. The effects of operational variables on the intrinsic rate of H-2 production, expressed per unit mass of catalyst per unit "transparency" of the reaction slurry, are investigated. It is shown that the intrinsic rate is a linear function of the intensity of incident illumination, an exponential function of the reaction temperature, and an exponential function of the steady-state pH value of the slurry. An equation describing the intrinsic rate of H-2 production over Pt-RuO2/TiO2 slurries is developed and values of the kinetic parameters are defined.