Ultrafine Zn-Fe composite oxides have been synthesized by the sol gel method and then modified by S=O. After preparation, these chalybeate compounds were characterized by BET surface area and porosity analyzer, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrometry (FT-IR). The influence of introduced S=O on the performance of ZnFe2O4 catalysts was measured, and the reaction mechanism was investigated. To evaluate the activities of different catalysts, a series of photo-Fenton reactions have been conducted to decontaminate the Reactive Black KN-GR simulated wastewater. Results indicated that the prepared catalysts were porous-material with a high specific surface area of 38 m(2) g(-1). Significantly, the FT-IR spectrum suggested that sulfated ZnFe2O4 calcinated at 773 K (named S773 catalyst) exhibited a strong acid property and XPS analysis proved the presence of S6+ (S=O), which led to the generation of acid sites. Under optimum conditions of 9.8 mM H2O2, 0.5 g L-1 catalyst, and pH = 6.0, up to 90% total organic carbon removal and 100% decolorization of a solution containing 100 mg L-1 Reactive Black KN-GR could be achieved after 150 min of treatment. The kinetics of acid catalysis were also discussed, and the stability of the catalyst was estimated on studying the iron leaching and S773 catalyst recycling.