CdS thin films of about 1 m thickness were deposited from an aqueous solution containing Cd²⁺, Na₂S₂O₃ and gelatin as the protective colloid to stabilize the size of colloidal sulfur at from 30 to 40 nm and keep the concentration to an appropriate value during electrolysis. The effects of concentrations of Cd²⁺ and S₂O₃ ^2– ions and the deposition potential on the composition of CdS films were studied. The reaction mechanism of CdS film formation on the electrode is discussed. CdS film, whose composition is uniform across the film and which does not contain excess metallic cadmium, can be deposited from a solution containing 0.50 to 2.00 mM Cd(NO₃)₂, 1.00 to 5.00 mM Na₂S₂O₃ and 1.0 × 10^–7 to 1.0 × 10^–3 wt % gelatin.