Zinc oxide (ZnO) and ZnO/CdS nanoparticles have been synthesized. The samples were characterized by Fourier transform infrared spectroscopy (FT–IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Photocatalytic reduction of toxic Hg (II) in aqueous solutions by suspension ZnONPs and ZnO/CdSNPs has been studies under an oxygen atmosphere. Flame atomic absorption (AFM) has been used to determine the concentration of Hg (II) in aqueous solutions. Photocatalytic reduction of Hg (II) was carried out with various ZnO nanoparticles (ZnONPs) concentrations. The optimum concentration of ZnO NPs was obtained at 0.24 mg/L. The highest photo reduction rate was obtained at 0.24 mg/ZnO/CdSNPs concentration. According to the experimental results, the reaction mechanism of ZnONPs and ZnO/CdSNPs illumination to produce oxidizing species and the effect of these species on the Hg (II) in aqueous solution has been suggested. Manuscript profile