Increasing environmental pollution caused by toxic dyes due to their hazardous nature is a matter of great concern. It has been generally agreed that reactive orange 5 (KGN) can be effectively degraded in aerated phosphotungstic acid (HPA) homogeneous reaction system using near-UV irradiation. The knowledge on the kinetics of the system possesses both practical and theoretical values. The results reveal that the photocatalytic degradation reaction of KGN with HPA in a homogenous solution can be described by Langmuir-Hinshelwood equation; the results manifest the reaction is first-order with lower concentration and is zero-order with higher concentration. As the initial concentration goes up, the order of the reaction is reduced from 1 to 0, the limiting rate constant and the adsorption constant in this case are0.8098 mg L-1 min(-1) and 4.359 x 10(-2) L mg(-1), respectively. The paper reveals that the optimal amount of photocatalyst in our experimental conditions for the degradation of KGN is 0.6 g L-1. The effects of irradiation intensity and pH value on the degradation rate constant were also demonstrated, where special attention was paid on the nature of the photocatalyst itself. (c) 2007 Elsevier B.V. All rights reserved.