Hydrogen peroxide and UV light were used in the decolourisation of azo dyes in spent dyeing wastewater. A kinetic study was carried out taking into account the contribution of the UV light alone and the combined reaction of UV light and H2O2. An empirical decolourisation rate expression was suggested by considering the two reactions taking place in parallel. Pseudo-first-order decolourisation rate constants were obtained front batch experimental data. Decolourisation rate was not significantly influenced by the initial pH of the wastewater in the range of 4-8. As the initial concentration of H2O2 increased, the rate of decolourisation increased. Consequently, the relationship of decolourisation rate constant (k(obs)) and initial concentration of hydrogen peroxide at constant UV light intensity was determined as k(obs) = 8.11 [H2O2](0)(0.63). Nearly completely decolourisation was achieved in 10 min with 5.88 x 10(-3) M H2O2 dose while only 18.9% of the organic carbon was mineralised. (C) 2003 Elsevier B.V. All rights reserved.