In this paper we study chemical bath deposition of ZnS thin films on glass slides with two different kinds of systems, that is, hydrazine hydrate system (N2H4•H2O/NH3•H2O/SC(NH2)2/ZnSO4) and citric acid system (C6H8O7/NH3•H2O/ SC(NH2)2/ZnSO4) in aqueous solution. The properties of ZnS thin films are characterized by X-ray diffraction, scanning electron microscopy and UV-Vis spectroscopy. The optical band gap is calculated from transmission spectra. The results show that ZnS thin films in citric acid system have better crystallization, higher transmittance and much lesser white spots which might be colloidal particles sedimentation mixed with ZnS, indicating that ZnS thin films prepared using citric acid system being more suitable for the buffer layer of CIGS solar cells.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.