The photolytic decomposition of the herbicide Bentazone (3-isopropyl-1H-2,1,3-benzothiadiazin-4-3H-one 2,2 dioxide) has been conducted by direct polychromatic UV radiation, and by the combination of that UV radiation with hydrogen peroxide. The experiments were performed at various temperatures and pH values, and the initial hydrogen peroxide concentration in the combined process was also varied. An Emission Model was used to evaluate the radiation flow rate absorbed by the reacting medium for each reaction time. From simple mechanisms, the rate equations for both processes were obtained. The application of the experimental data to those equations allows one to determine the quantum yields for the direct photolysis, and the kinetic constants for the reaction between Bentazone and the hydroxyl radicals generated in the combined photolysis.