Hydrochlorothiazide (6-chloro-1,1-dioxo-3,4-dihydro-2H-1,2,4-benzothiadiazine-7-sulfonamide) , a thiazidic diuretic very frequently used in the treatment of hypertension, has been oxidized by means of UV radiation, Fenton's reagent and ozone. The influence of some operating variables was established, and the main kinetic parameters were determined. Specifically, for the photodegradation process, quantum yields ranging between 0.028 and 0.048 mol Eins(-1) were obtained depending on the temperature and pH. At the same time, in the ozonation process and by using competition kinetics, the values deduced for the second-order rate constants varied from 91.3 M-1 s(-1) at pH = 3 to 16400 M-1 s(-1) at pH = 9. Similarly, competition kinetics was also used in the Fenton's reagent process for the determination of the rate constants for the reaction of this pharmaceutical with hydroxyl radicals, and the value deduced at 20 degrees C was (5.7 +/- 0.3) x 10(9) M(-1)s(-1). The oxidation of this selected pharmaceutical was also followed in some water systems (a surface water from a reservoir SW. and a secondary effluent from a municipal wastewater treatment plant SE), by UV radiation and ozone. The influence of the operating conditions on the removal efficiency was established, and specifically for the ozonation process, a kinetic model was proposed for the prediction of the elimination of the selected pharmaceutical in these water systems, which could well reproduce experimental data. (C) 2010 Elsevier B.V. All rights reserved.