Photoinduced absorption spectroscopy, PIAS, coupled with the measurement of dissolved O₂, % O₂, is used to probe the kinetics of water oxidation by periodate, photocatalysed by a P25 TiO₂ film on glass. The rate of the photocatalysed oxidation of water by periodate, r_O2, depends upon ρ^0.9, where ρ is the UV irradiance. In contrast, PIAS reveals the steady-state change in the absorbance of the film, ΔAbs_SS, is proportional to ρ^0.22, where ΔAbs_SS is a direct measure of the steady-state concentration of photogenerated holes on the surface of the photocatalyst particles, [h⁺]_SS. The combination of this data reveals r_O2 is proportional to [h⁺]_SS⁴, which is rationalised in terms of an electrochemical oxide path reaction mechanism with the combination of two surface adsorbed O atoms as the rate determining step. The potential of PIAS to provide an invaluable direct insight into the mechanism of photocatalytic reactions by powders is discussed briefly.