CdS:Cu nanocrystalline films were prepared by chemical bath on glass substrates at a deposition temperature of 80 S C. Different Cu-doping levels were obtained by changing the volume of the Cu-reagent-solution into the CdS growing solution. X-ray diffraction (XRD) and optical absorption (OA) measurements were carried out to characterize the material. From the XRD patterns it is concluded that grains in undoped films grow in the wurtzite hexagonal phase, and in Cu-doped films they grow in the zinc blende cubic phase. The average grain size, located in the range 14-23 nm, was calculated by employing Scherrer's formula. From OA spectra the forbidden energy bandgap (E-g) was determined by using the (alpha h nu)(2) proportional to (E-g-h nu) relation, where alpha is the OA-coefficient and h nu the photon energy. Due to the doping, Eg shifts to lower values depending on the impurity level of the film. Furthermore, the dependence of Eg with radius size and interplanar distances of the lattice is discussed. Gibbs free energy calculation for the Cu doping CdS process is also included. (c) 2006 The Electrochemical Society.