Response surface methodology (RSM) was applied to optimise the decolouration of the diazo, dye Reactive Black 5 (RB5) by crude laccase from the white-rot fungus Trametes pubescens. The presence of the redox mediator 1-hydroxybenzotriazole (HBT) greatly improved the decolouration levels of RB5 by crude laccase from T pubescens. Central composite design (CCD) using RSM with three variables namely redox mediator (HBT), dye (RB5) and enzyme (laccase) concentrations was used in this study to optimise significant correlation between the effects of these variables on the decolouration of RB5. The optimum concentrations of HBT, RB5 and laccase were 1.17mM, 150mg/l and 500U/l, respectively, for a maximum decolouration of RB5 (about 60% in 20 min). A quadratic model was obtained for dye decolouration through this design. The experimental values were in good agreement with the predicted ones and the model was highly significant, the correlation coefficient being 0.965. Interaction between HBT and RB5 concentrations, HBT and laccase concentrations and RB5 and laccase concentrations were negligible. In addition, when the kinetic parameters for RB5 decolouration were calculated according to Hannes-Wolf plot, the following values were obtained: K-M of 260.11 mg/l and V-max 37.59 mg/l min. (C) 2009 Elsevier B.V. All rights reserved.