The direct electrolytic reduction of solid SiO2 and an Ag+/Ag reference electrode in molten CaCl2 were investigated at 1123 K. An Ag+/Ag electrode separated by a mullite tube was confirmed to be a stable reference electrode. The electrochemical window of this melt was 3.27 V at 1123 K. The formation potential of Si-Ca alloys was measured. Combining these experimental data and the reported thermochemical data, a diagram was constructed which shows the equilibrium potentials for the silicon species in molten CaCl2 containing O2- ions at 1123 K, where the electrode potential vs Ca2+/Ca was plotted against the negative logarithm of the activity of oxide ions in the melt (pO(2-)). The potential-pO(2-) diagram explained the behavior of an SiO2-contacting electrode during the direct electrolytic reduction to silicon and suggested a formation of an intermediate of solid CaSiO3, which was confirmed by X-ray photoelectron spectroscopy analysis. (C) 2007 The Electrochemical Society.