The effect of the In/Cd ratio on both the microstructure and the electrical conductivity parameters of CdS thin films, as a good material for optical windows, was investigated. A new preparation method of In-doped CdS films was used. The effects of annealing time and annealing temperature as well as of thin-film thickness on the mentioned physical properties were studied. Qualitatively, the increase of the In content with increasing In/Cd ratio could be observed using energy dispersive analysis of X-ray (EDAX), a matter which may be used to confirm the validity of the preparation method. The annealing process was found to promote the conductivity of the investigated thin films. This can be attributed to a crystallization process as revealed by X-ray diffraction patterns and scanning electron microscope photomicrographs.