Cadmium sulfide (CdS) thin films were synthesized by chemical bath deposition on glass substrates at 80 A degrees C. The CdS thin films were doped with erbium (Er(3+)) during the growth process by adding aqueous solutions of Er(NO(3))(3)center dot 3H(2)O to the CdS chemical bath. The relative volume (V (r)) of the doping-solution was varied to obtain ten different doping levels. X-ray diffraction patterns displayed the zincblende crystalline structure for all the CdS:Er samples, with a remarked preferred orientation along the (111) direction. The (111) interplanar distance (ID) first decreased, reaching a minimum value at V (r) = 3%, after which point it increased up to saturation for largest values of V (r). The dependence of the band-gap energy (E (g)) on V (r) followed an opposite behavior to that described by the ID. The photoluminescence (PL) spectra showed essentially two main bands: the green emission (GE) band of CdS and a red emission (RE) band. The increasing of the PL signal, for V (r) a parts per thousand currency sign 4% has been associated with the presence of Er(3+) into the CdS lattice. For V (r) > 4%, Er degrades the lattice. In addition, the crystalline quality (CQ) and PL of the material improved for low V (r) values, which was in agreement with measurements of the mobility.