A synthetic non-steroidal anti-inflammatory drug diclofenac is one of the most frequently detected pharmaceuticals in various water samples. One of the most important degradation processes relating to diclofenac is its photodegradation in the aquatic environment. This paper studies the kinetic model for diclofenac degradation in water and the variation of the photodegradation of diclofenac in the presence of different forms of nitrogen changes with different pE values in the aquatic environment under simulated sunlight. The results demonstrate that degradation pathways proceed via pseudo first-order kinetics in all cases and the photodegradation of diclofenac is the sum of the degradation by direct photolysis and self-sensitization. NO(3)(-) and NO(2)(-) have inhibiting effects on the photodegradation of diclofenac. The different forms of nitrogen changes with different pE values and this has a significant influence on the photodegradation of diclofenac. The results show that when NH(4)(+) and NO(2)(-) coexist in the aquatic environment, the inhibiting effect on the photodegradation diclofenac is less than the sum of the partial inhibiting effects. The results indicated that NO(2)(-) had an obvious antagonistic action for NH(4)(+). When NO(3)(-) and NO(2)(-) coexisted in the aquatic environment, a similar antagonistic action between NO(3)(-) and NO(2)(-) was observed. (C) 2011 Elsevier B.V. All rights reserved.