Titanium dioxide (TiO2) photocatalysis is a possible alternative/complementary technology to conventional water treatment methods. The TiO2 catalyst may be used as slurry or it may be immobilised onto a supporting substrate. With immobilised TiO2 films mass transfer problems occur in most photocatalytic reactors, which results in a reduction of reactor efficiency and in the accuracy of measured catalyst efficiency and kinetics. In order to determine the real intrinsic kinetics of photocatalytic reactions on immobilised TiO2 films a stirred tank reactor (STR) was used. The reactor incorporated a propeller and a baffle, thus providing good mixing and efficient mass transfer to the TiO2 film. Degussa P25 was immobilised onto borosilicate glass by a dip coating method and the kinetics of the photocatalytic degradation of the model pollutants, formic acid and oxalic acid were investigated as a function of catalyst loading, initial pollutant concentration and propeller rotation speed. The rate of degradation. of either acid. was not mass transfer limited at propeller speeds greater than 1000 rpm. The rate of formic acid degradation was dependent upon catalyst loading up to a maximum loading above which a decrease in the degradation rate was observed. The apparent quantum yield for the photocatalytic degradation was 5% for oxalic acid and 10% for formic acid. This compares very well with usual reported apparent quantum efficiencies for photocatalysis which are similar to1%. The photocatalytic oxidation of both acids could be described using a Langmuir-Hinshelwood kinetic model.