ZnS　thin　films　were　deposited　on　glass　substrates　using　chemical　bath　deposition.　The　zinc　sulfate　and　thiourea　were　used　as　precursors　along　with　a　stable　complexing　agent　of　sodium　citrate　in　ammonia/ammonium　chloride　(pH=10.5)　buffer　solution.　The　ratio　of　Zn　and　complexing　agent　was　changed　from　6:1　to　1:1　by　varying　concentrations　of　the　complexing　agent.　X-ray　diffraction　(XRD),　scanning　electron　microscope　(SEM)　and　UV-vis　spectrophotometer　were　used　to　investigate　the　structure,　micrograph　and　optical　characteristics　of　the　ZnS　thin　films　respectively.　The　concentration　of　sodium　citrate　has　an　effect　on　the　crystalline　size　and　crystallization.　For　the　as-deposited　thin　films,　the　values　of　transmittances　and　Eg　are　about　85%　and　3.8　eV　respectively.　However,　they　are　decreased　to　75%　and　3.4　eV　respectively　after　annealing.　In　addition,　the　concentration　of　the　complexing　agent　has　no　remarkable　influence　on　both　the　transmittance　and　the　energy　gap.　The　results　show　that　the　ZnS　thin　films　with　resistivity　of　4.34×10　4　Ω·cm　are　suitable　for　optoelectronic　applications.　©　(2012)　Trans　Tech　Publications.