Assisted-photocatalytic degradation of the pesticide precursor 4-chlorobenzoic acid (4-CBA) in water was investigated using different TiO2 powders and near-UV radiation. Experiments were performed at pH=3.0 and at different TiO2 loadings, solution ionic strengths, and concentrations of hydrogen peroxide as an oxidant additive. Dark adsorption equilibrium studies revealed that surface area, ionic strength, and catalyst heterogeneity influenced the adsorption capacity and adsorption mechanism. This was attributed to the type and concentration of adsorption sites and the electrostatic interactions in the solution-semiconductor interface. However, the rates of the photocatalytic reactions were not significantly affected by an increase in the ionic strength by a factor of 50. On the other hand, the reaction rate was a strong function of catalyst crystallinity and loading. This suggested that the reactions were mostly controlled by the rate of formation of the oxidizing species rather than the extent of electric double layer (EDL) compression. Addition of hydrogen peroxide up to 248 mg/l resulted in an increase of the reaction rates with a corresponding increase in photonic efficiency by approximate to 20%. Above this concentration, hydrogen peroxide either did not enhance or caused a significant inhibition of the mineralization rates. (C) 2000 Elsevier Science B.V. All rights reserved.