Thin films of cadmium selenide with different thicknesses were prepared by the chemical deposition method from an alkaline bath using sodium selenosulphate as Se-2 ion source. The effect of film thickness on the optical, structural and electrical properties was studied. The shift of 0.5 eV in the optical band gap energy, E-g and a decrease in electrical resistivity from 10(4) to 10(3) Ohm cm and increase in the grain size of CdSe crystallites from 40 to 80 Angstrom were observed when the film thickness was varied from 600 to 2400 Angstrom. These changes are attributed to the quantum size effect in the semiconducting films.