The decolorization of C.I. Acid Red 27 (AR27), a monoazo anionic dye, was studied in the ultraviolet radiation (UV) alone and UV plus hydrogen peroxide (UV/H2O2) processes. The experimental results indicated that the kinetics of both oxidation processes fit well by pseudo-first order kinetics. The reaction rate was sensitive to the operational parameters and increased with increasing H2O2 concentration and light intensity. The reaction orders of H2O2 concentration and light intensity in both processes were obtained with linear regression method. A regression model was developed for pseudo-first order rate constant (k(ap,UV/H2O2)) as a function of the H2O2 concentration and UV light intensity. k(ap,UV/H2O2) = (2 X 10(-4) I-0(0.75) + k(3)I(0)(1.38) [H2O2](0)(n))phi(AR27) As a result of two opposing effects of H2O2 concentration at low and high concentrations, n has a value of 0.49 and -0.39 and k(3) has a value of 3 x 10(-4) and 0.1 for the regions of 0 mg l(-1) < [H2O2](0) < 650 mg l(-1) and 650 mg l(-1) < [H2O2](0) < 1500 mg l(-1), respectively. phi(AR27) is the initial dye concentration correlation index for developing of model for different initial concentrations of AR27. This rate expression can be used for predicting k(ap,UV/H2O2) at different conditions in UV alone and UV/H2O2 processes. The results show that UV alone cannot be an efficient method for decolorization of AR27 in comparison with UV/H2O2 process, therefore the first term of the model can be neglected. (c) 2006 Elsevier B.V. All rights reserved.