The structure of CdS thin films produced by chemical bath deposition for window layers for photovoltaic devices has been investigated using the techniques of standard theta-2 theta X-ray diffraction (XRD) and glancing angle XRD (GAXRD). The relative peal, intensities observed in the diffraction patterns were consistent neither with a cubic nor a hexagonal microstructure. By employing both diffraction geometries on the same sample it was established that the Cd layers were un-textured. thus a structure different from either the cubic or hexagonal modifications was inferred. In order to elucidate the crystalline structure of the material a Rietveld analysis program was employed, together with a special program written to generate appropriate input parameter files. It was found that a good match between experimental and simulated patterns could be obtained by postulating a polytype CdS structure, consisting of nearly random stocking sequences of the hexagonal planes that form the basis for both the cubic (zinc-blende) and hexagonal (wurzite) modifications. Thr GAXRD results indicate that little structural change occurs in the polytype CdS upon annealing in air, despite a pronounced colour change. The structural difference between films deposited under different conditions is much more pronounced than that between as-deposited and annealed material. The results are discussed with reference to investigations by other authors of CdS thin films and of CdS nanoparticles using thr techniques of transmission electron microscopy and/or X-ray or electron diffraction.