Diazinon is a widely used organophosphorus insecticide that is an important pollutant in aquatic environments. The chemical removal of diazinon has been studied using LTV radiation, ozone, Fenton's reagent, LTV radiation plus hydrogen peroxide, ozone plus hydrogen peroxide and photo-Fenton as oxidation processes. In the photodegradation process the observed quantum yields had values ranging between 2.42 x 10(-2) and 6.36 x 10(-2) mol E-1. Similarly, the ozonation reaction gave values for the rate constant ranging between 0.100 and 0.193 min(-1). In the combined systems UV/H2O2 and O-3/H2O2 the partial contributions to the global oxidation reaction of the direct and radical pathways were deduced. In the Fenton's reagent and photo-Fenton systems, the mechanism of reaction has been partially discussed, and the predominant role of the radical pathway pointed out. Additionally, the rate constant for the reaction between diazinon and the hydroxyl radicals was determined, with the value 8.4 x 10(9) L mol(-1) s(-1) obtained. A comparison of the different oxidation systems tested under the same operating conditions revealed that UV radiation alone had a moderate oxidation efficiency, which is enhanced in the case of ozone, while the most efficient oxidant is the photo-Fenton system. (c) 2007 Society of Chemical Industry.