This laboratory study was designed to investigate the removal of atrazine (ATZ) and its first main by-products, deethylatrazine (DEA) and deisopropylatrazine (DIA) by O-3/H2O2. At least 76% of the oxidation rate of atrazine is due to free radical reactions. At neutral pH and 20 degrees C, an initial hydrogen peroxide concentration of 10(-3) M is optimum to reach a maximum oxidation rate of these compounds. Experimental results of oxidation in the presence of high hydrogen peroxide concentrations allow the mass transfer coefficient of ozonation to be determined. This coefficient, reactor flow analysis and kinetic data obtained have been applied to mol balance equations of atrazine, deisopropylatrazine, deethylatrazine, ozone (both in the gas and water) and hydrogen peroxide to obtain their corresponding concentrations at different conditions.