The photocatalytic oxidation of formate ion at illuminated TiO2 particles was investigated in an annular slurry reactor and in a rotating disk reactor that utilized TiO2 particulate films. Rates of CO2 formation were determined during continuous and periodic illumination over a range of oxygen and formate ion concentrations. These investigations indicated that the periodic illumination effects reported for this reaction, and for other photocatalytic oxidations, resulted from intraparticle diffusion in flocculated particles, mass transport of oxygen to the catalyst surface, slow or weak adsorption of formate ion, or a combination of these processes.