Disinfection studies with two pure strains of Escherichia coli ATCC 11229 and ATCC 15597 and real municipal wastewater were carried out at laboratory scale with a 10 W low pressure ultraviolet lamp in an ultraviolet (UV) disinfection system with recirculation of the liquid. A double first order kinetic model was established to fit the ultraviolet disinfection curves for the two microorganisms. This model implies that there are two subpopulations within the study population, one of which is sensitive to ultraviolet and another that is radiation-resistant. The kinetic model depends on the applied UV dose and is directly related to the initial bacterial concentration (N-o). It was possible to establish relationships between the kinetic parameters of the equation and the initial concentration, and so to predict the disinfection curves. The kinetic model was successfully fitted to a real wastewater sample, with the final microbial disinfection predicted as a function of the UV dose and N-o. (C) 2011 Elsevier B.V. All rights reserved.