Photodegradation of textile dyestuffs Chrysophenine Yellow (DY12), Congo Red (DR28) and Remazol Black B (RB5) by the UV/H2O2 process was investigated in a laboratory scale photoreactor equipped with a 16-W low-pressure mercury vapour lamp. The Box-Wilson experimental design was employed to evaluate the effects of major process variables, dyestuff and hydrogen peroxide concentrations and irradiation time on decolorization efficiency. Response function coefficients were determined by regression analysis of experimental data and predictions were found to be in good agreement with the experimental results. For 100 mg l(-1) DY12, RB5 and DR28 for instance, the optimum hydrogen peroxide concentration and irradiation time were found to be 45 mM for 58 min, 45 mM for 58 min and 60 mM for 120 min with 100, 100 and 85.7% efficiencies, respectively. Further increases in oxidant concentration led to slight decreases in decolorization efficiencies according to the response function, most probably due to the scavenging effect of excess hydrogen peroxide on hydroxyl radicals. And this was confirmed by the repeated experiments performed at the points of concern. lncreasing the dyestuff concentration also decreased the decolorization efficiency even at higher concentrations of oxidant and longer irradiation times. The photodegradation of dyestuffs were found to follow a first-order reaction law. (C) 2003 Elsevier Ltd. All rights reserved.