Abstract: The kinetics of photooxidation to CO2 of 22 organic solutes over a UV illuminated film of Degussa P25 titanium dioxide have been studied over a 100-fold concentration range for each solute, generally from about 1 to 100 mg liter−1. The dependence of the photooxidation rate on concentration obeyed a simple two-coefficient Langmuir expression for each solute. The coefficients reflecting the degree of adsorption on the TiO2 and the limiting rate at high concentrations enable the prediction of photocatalytic rates in any TiO2-based photoreactor of a similar type once a reference rate has been determined for one of the solutes. A reaction mechanism is proposed for the oxidation of aromatic compounds involving peroxyhydroxycyclohexadienyl- and mucondialde-hyde-type compounds as important intermediates. The solutes studied were benzoic acid, salicylic acid, phenol, biphthalate, 2-chlorophenol, 4-chlorophenol, monochlorobenzene, nitrobenzene, methanol, ethanol, n-propanol, 2-propanol, acetone, ethyl acetate, acetic acid, formic acid, sucrose, 2-naphthol, umbelliferone, chloroform, trichloroethylene, and dichloroethane.