In this study, CdS layer in CdS–TiO2 bilayer films was fabricated by two different chemical bath deposition techniques: one with an ammonium complex as complexing agent and the other with sodium citrate as a substitute for the ammonium complex. The TiO2 upper layer was deposited by dc magnetron sputtering technique for both the CdS underlayers. The effect of postdeposition vacuum annealing was studied for both samples. The photocatalytic activity was evaluated by measuring the decomposition of methanol in gas phase under ultraviolet-visible irradiation. The CdS thin films, prepared by both techniques, show a highly oriented hexagonal structure. The optical transmission edge of the CdS thin films deposited by the ammonia-free technique is in lower wavelength than that of the other technique. For both techniques, vacuum annealing extends the absorption edge to higher wavelength, but this effect is more pronounced for the ammonia-content technique. The decomposition efficiency of the as-deposited CdS–TiO2 thin film prepared by the ammonia-free technique is much higher than the as-deposited CdS–TiO2 prepared by ammonia-content technique. In case of the ammonia-content technique, high-vacuum annealing improves the photocatalytic activity but it has minimal influence in the other technique. The surface morphology of all the CdS thin films was studied by field emission scanning electron microscopy and atomic force microscopy in order to find out the possible inherent reason that leads to a huge difference in the photocatalytic decomposition activity of CdS–TiO2 thin films.