The degradation of the organic content of a bleaching Kraft mill effluent (BKME) has been carried out by using Fenton reagent and irradiation providing the conditions needed for the simultaneous occurrence of Fenton and photo-Fenton reactions. The main parameters that govern the complex reactive system, i.e. light intensity, temperature, pH, Fe(II) and H2O2 initial concentrations, and O-2 presence in solution have been studied. Concentrations of Fe(U) between 0 and 800 ppm, and H2O2 between 0 and 10,000 ppm were chosen. Temperatures above 25 and up to 70 degreesC have a beneficial effect on total organic carbon (TOC) decay rate. The presence of small amounts of O-2 seems to be enough to ensure the reaction progress. The combination of Fenton and photo-Fenton reactions has been proved to be highly effective for the treatment of such a type of wastewaters, and important advantages concerning the application of this combination of reactions arise from the study.