As the first stage for thin film preparation of copper indium disulfide (CuInS2), an electrodeposition technique of thin films in the Cu-S system was investigated from a new viewpoint. Deposition was carried out potentiostatically on a Ti substrate from acidic aqueous solution containing CuSO4 and Na2S2O3. Tartaric acid was found to be effective as a buffer for stabilizing the hydrogen ion concentration. Thin films of Cu2S were obtained at -0.7 V vs. Ag/AgCl with good reproducibility from a solution containing 10 mM CuSO4, 400 mM Na2S2O3 and 100 mM tartaric acid. Scanning electron microscopy and energy dispersive X-ray analyses revealed that the film deposited had a crack-free surface and uniform stoichiometry of Cu2S. Film thickness was estimated to be 0.6-0.8 x 10(-6) m after 3600 s deposition. The mechanism of Cu2S formation was supposed to be that S2O32- ion reduces Cu(II) ion to make complex with Cu(I) ion. Probably it is this complex that contributes to the Cu2S formation.