In this work, UV/H2O2 treatment of actual pulp and paper mill wastewater (PPMW) was investigated. To determine optimal conditions, the effects of some experimental parameters (initial pH, initial H2O2 concentration, temperature, and organics load) on the UV/H2O2 process efficiency in terms of absorbance at 330 and 281 nm and chemical oxygen demand (COD) and total organic carbon (TOC) removals were studied. The results show that the UV/H2O2 system leads successfully to the almost-complete elimination of absorbance at 330 and 281 rim, COD, and TOC from an actual PPMW containing 270 mg of C L-1 under natural pH (pH 11.25) using 2.1 g of H2O2 L-1 at 28 degrees C. Under these optimal conditions, pseudo-first-order kinetics was determined for absorbance at 330 and 281 nm and COD decay with rate constants of 0.029, 0.02, and 0.008 min(-1), respectively. Furthermore, the evolution with time of the H2O2 concentration exhibits two parts: a linear decrease to 300 min and then exponential change until the end of the treatment. This shows that the kinetics of H2O2 disappearance changes during the treatment from a pseudo-zeroth-order reaction in the first steps to a pseudo-first-order reaction in the final steps. On the other hand, a continuous decrease of pH and a rapid total phenols disappearance were observed during UV/H2O2 treatment of PPMW. These results suggest that several steps are involved in the photochemical oxidation of organics, starting with degradation of lignin derivatives and tannins to obtain aromatic intermediates that then undergo oxidative ring opening, leading to aliphatic carboxylic acids. In the final steps, carboxylic acids are mineralized into CO2 and H2O.