In a previous study it has been shown that the combination of hydrogen peroxide and UVC radiation is an effective and feasible process to oxidize arsenic in water. In this work, a kinetic model for the photo-oxidation of As (III) is presented. An initial concentration of 200 mu g/L of As (III) was used. The model was based on mechanistic reaction steps and included the modeling of the local radiation absorption effects. The evaluation of the radiation field inside the reactor was achieved by solving the radiative transfer equation for the homogeneous system. Both dark and photochemical arsenic oxidation rates were incorporated in the complete kinetic expression. Three parameters were estimated and a simplified model that only considered photoactivated reactions was developed. The rates obtained from these simplifications were compared with the rates from the complete model and showed a satisfactory concordance. The study was carried out in a cylindrical reactor operated in batch recirculation mode and two germicidal lamps were used (lambda = 254 nm) as the source of radiation. Experimental runs were performed varying the hydrogen peroxide concentration and incident radiation. Both models rendered a good representation of the experimental data. (C) 2012 Elsevier B.V. All rights reserved.