The electrochemical reduction of borosilicate glass in molten CaCl2 at 1123 K was investigated. The behaviors of the constituent elements, i.e., Si, B, Al, Na, and K, were estimated using potential-pO(2-) diagrams constructed from the thermodynamic data for the species. The diagrams suggested that the first cathodic wave in the cyclic voltammogram results from the reduction of the B2O3 component. The dissolution of the Na2O and K2O components, which was predicted from the diagrams, was confirmed by energy dispersive X-ray analysis of a borosilicate glass plate after immersion into molten CaCl2 without electrolysis. The scanning electron microscopy/wavelength dispersive X-ray mappings and Raman spectrum for borosilicate glass reduced at 0.9 V vs. Ca2+/Ca indicated that SiO2 and B2O3 are reduced to Si and B-Si compound. The formation of Ca-Al-O compounds owing to the increase of O2- ions is suggested. The pO(2-) range during electrolysis at 0.9 V was indicated to be 2.95-3.46. (C) The Author(s) 2017. Published by ECS. All rights reserved.