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Cu -doped Cadmium sulfide ( CdS ) films were prepared by chemical bath deposition method. Various quantities of Cu were used for mixing. Surface characterization, structure, morphology and defect structures of the films were studied. The structure of all Cu -doped CdS films was a unique CdS cubic phase. The grain size of CdS was decreased when doped with Cu . The F-type defect in the undoped CdS is clearly confirmed by an ESR signal arising from the hyperfine interaction of electron spin (S=1/2) trapped in sulfur vacancies and neighboring cadmium nuclear spin (I=1/2). In the Cu -doped CdS films, the ESR peaks shift towards low fields as the copper concentration is increased. This is due to the change in crystal field experience by Cu 2+ (3 d 9) ions while the Cd 2+ signal disappears. The Cu 2+ ions in the Cd 1-x Cu x S itself are ESR silent due to a very short relaxation time. The dark resistance increased with increasing amount of Cu concentrations up to about 0.03 M. This 0.03 M Cu -doped CdS sample also possesses the maximum photosensitivity.