An approach was developed for electrochemical codeposition of In-Se and Ga-Se films with high repeatability and controllable In/Se and Ga/Se molar ratios to prepare precursor layers for copper indium gallium diselenide (CIGS) film formation by two-stage processes. Full potential of complexation was used for the first time by using aqueous electroplating solutions containing complexing agents at alkaline regime. It was found that tartrate and citrate were suitable complexing agents to solubilize In and Ga ions at high pH, respectively. Because no appreciable complexation of Se occurred, Se reduction potential could be independently controlled by the amount of dissolved Se. Use of alkaline solutions also reduced hydrogen bubble generation and related defects.