EMF measurements were carried out at temperatures ranging from 1280 to 1490 K in the following cells: - Mo, Si + NbSi2 + SiO2/SiO2-sat. Li2O-SiO2/NbSi2 + Nb5Si3 + SiO2, Mo +, - Mo, Si + NbSi2 + SiO2/SiO2-sat. Li2O-SiO2/Nb5Si3 + NbO + SiO2, Mo +, and - Mo, NbSi2 + Nb5Si3 + SiO2/SiO2-sat. Li2O-SiO2/Nb5Si3 + NbO + SiO2, Mo +, using SiO2-saturated lithium silicate liquid electrolyte. Each of the cells showed a reliable electromotive force (emf) corresponding to the difference in silicon potential between the electrodes. Based on these emf values measured, the molar standard free energy of formation for NbSi2 and Nb5Si3 were determined to be Delta(GNbSi2)(o)/kJ = -165 + 0.008 (T/K) +/- 13 and Delta(GNb5Si3)(o)/kJ = -526 + 0.009 (T/K) +/- 63, respectively. (C) 2003 The Electrochemical Society.