Ozonation of sulfamethoxazole promoted by a powder activated carbon (PAC) of neutral nature has been studied for mechanistic and kinetic purposes. The main advantage of using powder activated carbon was the removal of both liquid-solid and internal diffusion mass transfer resistances. In addition, at the operational conditions applied the influence of the gas-liquid mass transfer was eliminated. As a result, in the absence of any physical mass transfer resistances, a mechanism that involves both homogenous and heterogeneous reactions for the mineralization of the water matrix was proposed and a mathematical kinetic model was obtained from the corresponding mass balances of main species present. In order to properly solve the kinetic model a term indicative of the changing nature of the water matrix was introduced. In that way the promoting or non-promoting character of water constituents was considered in the mechanism. (C) 2010 Elsevier B.V. All rights reserved.