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Cadmium sulphide (CdS) thin films deposited on indium tin oxide (ITO) substrates were prepared by the chemical bath deposition technique at different values of pH and pNH 3 . This was made in order to analyze the influence of the initial chemical composition of the bath on some physical properties of the final CdS films, used as optical windows for solar cells. The mechanism proposed in the literature for this deposition involves the tetra-ammonium cadmium (II) complex, Cd(NH 3 ) 2+ 4 , at the start of the reaction. Following this mechanism, the change in the concentration of Cd(NH 3 ) 2+ 4 gives a variation in the rate of deposition with the corresponding modification in the quality of the film. Therefore, the predominance zone diagram of Cd 2+ species in solution as a function of the values of pH and pNH 3 was used to analyze the reasons why the CdS/ITO thin films are favored in just a narrow range of ammonium and hydroxide concentrations. To obtain useful films, the results showed that low ammonium concentrations must be avoided at high pH values as well as high ammonium concentrations at lower pH values.