We prepared pure and samarium-doped ZynS (SmxZn1-xS1+0.5x) nanoparticles via hydrothermal process at 160 degrees C for 24 h. XRD analysis shows that the particles were well crystallized and corresponds to a cubic sphalerite phase. SEM and TEM images indicate that the sizes of the particles were in the range of 20-60 nm. The photocatalytic activity of Sm-doped ZnS nanoparticles was evaluated by monitoring the decolorization of Reactive Red 43 in aqueous solution under visible light irradiation. The color removal efficiency of Sm0.04Zn0.96S and pure ZnS was 95.1% and 28.7% after 120 min of treatment, respectively. Among the different amounts of dopant agent used, 4% Sm-doped ZnS nanoparticles indicated the highest decolorization. We found that the presence of inorganic ions such as Cl-, CO32- and other radical scavengers such as buthanol and isopropyl alcohol reduced the decolorization efficiency. (C) 2015 Elsevier Ltd. All rights reserved.