Performance of an advanced oxidative process (AOP) with the application of hydrogen peroxide and UV radiation (H2O2/UV) has proven effective to inactivate Giardia duodenalis cysts. Hydrogen peroxide concentrations of 15 mg. L-1 and 6x10(3) mg. L-1 and 44 UV doses of 2,736 and 5,472 mW s. cm(-2) were used in this study. The process effectiveness was assessed observing the occurrence of damage to the cyst's wall using direct immunofluorescence reaction (IFA) and scanning electron microscopy (SEM). When using the lower UV radiation dose and the lower oxidant concentration the AOP has shown higher effectiveness both in causing damage to the Giardia duodenalis wall and in reducing the number of cysts inoculated at the beginning. The average reduction AOP and peroxidation processes reduced 45.2 and 22.6% in the number of G. duodenalis cysts and 42 and 29.4% of the remaining cysts presented damage, respectively.202 159164