[en] A high quality lanthanum (La) doped cadmium sulfide (CdS) thin films were synthesized onto a glass substrates by chemical bath deposition. The effects of lanthanum doping concentration on the composition, structural and optical properties of CdS thin films have been studied. The transmittance of CdS thin film is 80% and gradually decreased as the La content was increased. Analysis of the optical absorption coefficient of CdS and La doped CdS thin films revealed that a presence of direct optical transition and the optical direct band gap of the films apparently decreased from 2.45 to 2.09 eV when the lanthanum content was increased from 0 to 12 wt %. The values of optical and electrical conductivity of CdS and La doped CdS thin films have been increases with increasing the lanthanum content. The self cleaning test shows that 12% La doped CdS thin films have excellent photocatalytic activity. (paper)