One-step electrodeposition using sodium thiosulfate (Na2S2O3) as a sulfur source has been studied for the preparation of Cu-In-S thin films. A deposited film is found to have a sufficiently high sulfur content compared with films deposited using thiourea as a sulfur source. The film deposited using Na2S2O3 is also found to have an excellent morphology compared with electrodeposited Cu-In precursors, Predominant factors to govern film composition, In/Cu and S/(Cu + In) ratios, are also investigated in this study. An HCl content added in order to decompose S2O32- ions in the solution is found to be one of the important factors to control composition of deposited films. A sulfur cocentration in the solution influences not only S/(Cu + In) ratio but also In/Cu ratio in the film. Reproducibility of film composition is deteriorated as the solution temperature increases.