Pure and Eu-doped ZnO nanostructures with different amounts of Eu were synthesized using a simple sonochemical method. The as-synthesized samples were characterized by X-ray diffraction, scanning electron microscopy, UV-Vis spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The photocatalytic activity of as-prepared Eu-doped ZnO was investigated based on the decolorization of Acid Red 17 (AR17) under visible light irradiation. The decolorization efficiency was found to be 25.2%, 55.6%, 63.1% and 30.3% for undoped, 1% Eu-ZnO, 3% Eu-ZnO and 5% Eu-ZnO, respectively. Using 3% Eu-doped ZnO nanoparticles, the decolorization efficiency reached 100% by adding 0.1 mM S2O82-. Response surface methodology based on five-level central composite design was applied to model and optimize the decolorization of AR17 over 3% Eu-doped ZnO/S2O82-. Accordingly, the value of correlation coefficients (R-2 = 0.970 and adjusted-R-2 = 0.935) obtained from analysis of variance confirmed the adequacy of fitted model. The maximum decolorization efficiency of 99% was achieved at an initial dye concentration of 5 mg/L, catalyst dosage of 1.25 g/L, S2O82- concentration of 0.25 mM and reaction time of 150 min. The decolorization efficiency decreased only 7% after 4 repeated runs. (C) 2014 Elsevier B.V. All rights reserved.