A facile and low-cost approach was developed to prepare high quality quaternary Cu₂ZnSnS₄ (CZTS) alloys through the thermal treating of the core–shell Cu₂SnS₃@ZnS nano particles. The synthesis process of the core–shell structure involves a simple two-step hydrothermal route that starts from the well- dispersed ternary Cu₂SnS₃ core with the introduction of poly(vinylpyrrolidone) (PVP) in the reaction solution, and ended by the coated binary ZnS shell upon the adsorption of citrate groups onto the preformed cores. X-ray diffraction, Raman spectrum and UV–vis absorption spectrum confirmed the single-phase formation and the proper bandgap of 1.51 eV for the CZTS alloys. This effective hydrothermal method can be readily applied for industrial production.