The effect of the presence of formic, oxalic and acetic acids on photocatalytic degradation of p-nitrophenol has been studied. Adsorption and FTIR studies indicated that the presence of such acids modifies p-nitrophenol interaction with TiO2 surface, which is more rapidly degraded. The higher degradation rate constants have been correlated with interaction changes and radicals generated in the degradation of carboxylic acids. The presence of H2O2 did not affect the mechanism responsible for the faster p-nitrophenol degradation in the presence of acetic or oxalic acids. However, formic acid seems to hamper the production of OH center dot radicals from H2O2 degradation. (c) 2007 Elsevier B.V. All rights reserved.