A mechanistic model of the inactivation of Escherichia coli by solar water disinfection (SODIS) technique is presented. Bacterial inactivation by SODIS is commonly attributed to the oxidative stress generated by synergy among solar radiation (UV photons) and mild temperature. Photons may increase the naturally occurring amount of internal Reactive Oxygen Species (ROS), such as hydroxyl radical (HO center dot) and superoxide radical (O-2(center dot-)). ROS attacks to different targets inside the cells are one of the main sources of oxidative damage over cells. Besides, photons may damage the two essential enzymes of the defense system against intracellular oxidative stress, catalase (CAT) and superoxide dismutase (SOD). Therefore, the proposed model is a simplified approach of the complex processes occurring inside cells during SODIS, which is based on the photo-induced formation of intracellular ROS and the photo-inactivation of CAT and SOD. The model considers two individual volume units in which the processes are occurring simultaneously: (i) a single cell (mass balances for intracellular ROS and enzymes) and (ii) the reactor (mass balance for bacteria). Kinetic constant from literature were used, meanwhile CAT photo-inactivation kinetic constant was determined experimentally, (1.50 +/- 0.04).10(7) cm(3) Einstein(-1). Model regression was done using experimental data of E. coli inactivation by solar disinfection at different controlled conditions of solar irradiance and initial bacterial concentration. The good fit of the simulated and experimental results suggested that the mechanistic process proposed is a realistic approach of the disinfection process. Moreover, simulations of the time profile of intracellular ROS and enzymes involved during bacterial inactivation by SODIS are also presented. (C) 2016 Elsevier B.V. All rights reserved.