The present study investigates the degradation of atrazine (2-chloro-4-(ethylamino)-6-isopropylamino-s-triazine) by ozone and OH radicals during ozonation and advanced oxidation processes, with the identification of the main degradation products. Besides the dealkylated and amide degradation products(6-amino-2-chloro-4-isopropylamino-s-triazine, 6-amino-2-chloro-4-(ethylamino)-s-triazine, 4-acetamido-2-chloro-6-isopropylamino-s-triazine, 4-acetamido-6-amino-2-chloro-s-triazine, and chlorodiamino-striazine), two new degradation products with an imine group were identified (2-chloro-4-ethylimino-6-isopropylamino-s-triazine and 6-amino-2-chloro-4-ethylimino-s-triazine). The contribution of the different pathways (direct ozone and OH radical reaction) to the overall degradation process has been quantified, and the rate constants of the reactions of atrazine and its main degradation products with both oxidants have been measured. The ethyl group is more reactive than the isopropyl group (i.e. 19 times during ozonation and four times during OH radical attack). The ethyl group reacts in higher proportion through oxidation to acetamide or imine derivates than to dealkylation. In contrast, the isopropyl group reacts mainly through dealkylation to the free amino group. Acetamido and imino groups a re found to be resistant to chemical oxidation. These reactivities were corroborated by the measured values of the rate constants with both oxidants. A combination of product distribution and the kinetic parameters together with ozone and OH radical concentrations allowed us to calculate the evolution of the concentration of the degradation products for a given ozonation process.