Zinc sulphide thin films have been deposited on glass substrates using the chemical bath deposition technique. The depositions were carried out in the pH range of 10 to 11.5. Structure of these films was characterized by X-ray diffraction and scanning electron microscopy. Optical properties were studied by spectrophotometric measurements. influence of the increased pH value on structural and optical properties is described and discussed in terms of transmission improvement in the visible range. Transmission spectra indicate a high transmission coefficient (similar to 70%). The direct band gap energy is found to be about 3.67 eV for the films prepared at pH equal to 11.5. (c) 2005 Elsevier B.V. All rights reserved.