An internally-irradiated annular photoreactor has been used to investigate the oxidative degradation of aqueous 4-nitrophenol with titania as the photocatalyst. Reaction runs were performed over a 3-h period and in practically all cases, complete degradation was possible within about 2h. The kinetics was determined as a function of nitrophenol concentration, oxygen partial pressure, catalyst loading, pH, temperature and light intensity. The reaction was characterised by a relatively low activation energy of 7.83 kJ mol(-1) although transport intrusions were negligible. Rate decreased almost exponentially with pH while a quadratic (maximum) behaviour with respect to both oxygen pressure and nitrophenol concentration is symptomatic of self-inhibition possibly due to the formation of intermediates which competitively adsorb on similar sites to the reactants. Increased catalyst dosage also improved the reaction rate although the possible effects of light scattering and solution opacity caused a drop at loadings higher than about 1.20 g dm(-3). Rate, however, has a linear dependency on light intensity, suggesting that hole-electron recombination processes were negligible at the conditions investigated.