Homogeneous (H2O2)and heterogeneous (TiO2 + Oz)aquatic photochemical systems were compared in their ability to generate hydroxyl radicals,ana to decompose-p-toluenesulfonic acid (PTS) in buffered and unbuffered solutions. The kinetics of hydroxyl radical formation were monitored with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap. The zero zeta potential measured in aqueous TiO2 suspensions was-found at pH = 6, but zeta potentials shifted to considerably more negative values in phophate-borate buffers. Consequently, a well-defined PTS adsorption isotherm on the TiO2 surface was found in unbuffered systems, and no PTS and DMPO adsorption was measurable in the phosphate-borate buffer, due to the competitive adsorption of buffer ions. The identical dependence of (.)DMPO-OH formation on PTS concentrations in both homogeneous and heterogeneous buffered systems along with zeta potential and adsorption measurements suggests that the reaction of (OH)-O-. radicals, their addition to DMPO and the oxidative degradation of PTS, is taking place (in the presence of phosphate-borate buffer) in the homogeneous phase, with radicals leaving the TiO2 surface.