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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 …