Monuron, 3-(-chlorophenyl)-1,1-dimethyl urea, is one of the most important phenylurea herbicides. It is somewhat more soluble in water (230 mg/L at 293 K) than Diuron and has a lower coefficient of adsorption. Therefore, more than 98% of the applied substance is not used to kill the target weeds but dispersed in the different environmental compartments, mainly in aquatic environment. Monuron has a half life of more than 56 days in river water and equal to 170 days in soil. It is a persistent and bio recalcitrant pollutant. The present paper deals with the study of the Monuron degradation and mineralization in water by three Advanced Oxidation Processes (A.O.P.): UV/H2O2, UV/H2O2/ Fe2+ (photo- Fenton), and UV/H2O2 /TiO2. All the degradation experiments have been carried out in a quartz glass photo-reactor (V= 0.5 L) equipped with a UV lamp having an incident photonic flux of 5x10-6 Es/s. The initial concentration of the Monuron aqueous solution was 2x10-4 mol/L. The concentrations of the unreacted substrate and of some intermediates have been determined by HPLC analysis. The Total Organic Carbon (TOC) measurements have been performed with a TOC analyzer Shimadzu (model TOC 5000A). The optimal parameters (pH, H2O2 / Fe2+ /TiO2 doses) have been identified for each process. The main conclusion from the obtained results is that the most effective is the photo-Fenton process. Using doses of 20 mmol/L H2O2, and 1 mmol/L Fe2+ the Monuron is almost completely removed after only 15 minutes while TOC is reduced by 50% after 60 minutes of irradiation.