Silicon (Si)/carbon (C) composites, silicon carbide (SiC), and Si films are electrochemically prepared in molten CaCl2, and the interaction of Si with C is systematically analyzed. Further, the effects of the cell voltage of electrolysis and the formation of SiO2 and C on the electrolytic products were studied. Electrolytic SiC can be identified as the thermodynamically favored product because the Gibbs free energy of formation of SiC is negative at 850 degrees C. The electrolysis of the C/SiO2 mixture prepared by the mechanical mixing of the C (i.e., graphite or sugar-derived C) and SiO2 powders resulted in the generation of the thermodynamically unfavored Si/C composite. Further, the thermodynamically favored SiC was obtained by the electrolysis of SiO2 coated with sugar-derived C. Moreover, the electrodeposition of Si films onto both graphite foils and C fibers was conducted in molten CaCl2, and the boundary of the electrolytic Si film/C substrate was obtained using scanning electron microscopy and X-ray photoelectron spectroscopy. Overall, the contact area between SiO2 and C is required for engineering the electrolysis to obtain products such as the Si/C composites, SiC, and Si films. (C) 2019 The Electrochemical Society.