The simultaneous application of microwave (MW) power and UV light leads to improved results in photochemical processes. This study investigates the oxidative decomposition of atrazine in water using an innovative MW and UV photochemical reactor, which activates a chemical reaction with MW and UV radiation using an immersed source without the need for a MW oven. We investigated the influence of reaction parameters such as initial H2O2 concentrations, reaction temperatures and applied MW power and identified the optimal conditions for the oxidative decomposition of atrazine. Atrazine was completely degraded by MW/UV/H2O2 in a very short time (i.e. t(1/2) = 1.1 min for 20.8 mg/L in optimal conditions). From the kinetic study, the disappearance rate of atrazine can be expressed as dX/dt = k(PH)[M](0)(b - X)(1 - X), where b equivalent to[H2O2](0)/[M](0) + k(OH)[(OH)-O-center dot]/k(PH)[M](0), and X is the atrazine conversion, which correlates well with the experimental data. The kinetic analysis also showed that an indirect reaction of atrazine with an OH radical is dominant at low concentrations of H2O2 and a direct reaction of atrazine with H2O2 is dominant when the concentration of H2O2 is more than 200 mg/L (C) 2010 Elsevier B.V. All rights reserved.