Thin films of zinc sulphide (ZnS) were deposited on glass substrate using the chemical bath technique. The films were grown at different bath temperatures in the range 70 °C to 85 °C, with other deposition variables (complexing agent, pH, source to substrate distance, etc) kept constant. The deposited films were annealed at annealing temperatures ≤200 °C for one hour. The effect of the different bath temperatures and annealing treatments on the films were investigated with emphasis on the structural and optical properties. X-ray diffractometry (XRD) was used to investigate the crystal structure and phases while UV-Spectrophotometer was used for measurement of transmittance and reflectance spectra, enabling to deduce the important optical constants. The results indicate that the different bath temperatures has profound influence on the structural properties and the optical constants. In particular, the crystallite size varied between 34 nm to 45 nm, exhibiting an increase with increased bath temperatures for as grown layers and exhibited an optimum of 39 nm to 47 nm for the annealed layers. The dislocation density was between 4.9 × 1016 lines m−2 to 8.5 × 1016 lines m−2. The post-deposition annealing resulted in increased texturing in the films. The bandgap obtained were direct, and varied between 3.60 eV to 3.78 eV indicating a blue shift in the band gap with respect to the increasing bath temperatures. However, annealing the films at an annealing temperature of 200 °C resulted in an increase of the energy bandgap from 3.80 eV to 3.95 eV. This was attributed to the re-evaporation of sulphur due to the post-deposition heat treatments. The refractive index was between 1.25–2.6 for the as-grown layers and increased from 1.40 to 4.5 for the annealed layers. The extinction coefficient varied between 0.20–1.20 for the as deposited films and 0.16–0.65 for annealed films. The electrical resistivity of the layers was found to be in the order of 103 Ωcm for both as-grown and annealed layers. The values of the energy bandgap strongly suggest the use of the films in various optoelectronic devices especially as window layers in solar cell devices as this will permit more light to reach the absorber layers and hence increase the solar conversion efficiency.