The effect of hydrogen peroxide on the photocatalytic degradation of organic contaminants in water was investigated using a TiO2-rotating disk photocatalytic reactor (RDPR) operated in a continuous-mode and at steady state. The experiments were performed at pH 3.0, in the presence of near-UV radiation, and using 4-chlorobenzoic acid (4-CBA) as a model non-volatile organic contaminant at influent concentration of 300 mumol 1(-1). Experiments were performed at concentrations of hydrogen peroxide in the range 0-10.74 mmol 1(-1). Addition of hydrogen peroxide at small concentrations (<2 mmol 1(-1)) had a synergistic effect and increased considerably the rates of photocatalytic reactions. An optimum influent hydrogen peroxide concentration was observed at 1.6 mmol 1(-1), which caused an increased in the rates of 4-CBA degradation and total organic carbon (TOC) mineralization by 1.72 and 2.13 times, respectively. This corresponded to an optimum oxidant to contaminant molar ratio of 5.33. At higher concentrations, hydrogen peroxide was found to cause an inhibiting effect on the photocatalytic reactions. The synergistic and inhibiting effects of hydrogen peroxide were rationalized based on the reaction rate constants between relevant radical species. (C) 2004 Elsevier B.V. All rights reserved.