The photo-oxidation of 23 compounds (benzene, haloaromatics, chloroalkenes, haloalkanes, bis-(2-chloroethyl)ether, 1-hydroxyethane-1, 1-diphosphonate and ethylenediamine-N,N,N′,N′-acetic acid) in aqueous solution with titanium dioxide as the photocatalyst was investigated. The plots of photo-oxidation rate vs. irradiation time were analysed on the basis of a simple kinetic model for homogeneous reactions. This model yielded formal rate constants which serve as a general measure for the efficiency of photo-oxidation.