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The stability of Ag│Ag+ molten chloride reference electrodes was studied using mullite and magnesia tubes as reference electrode membrane materials in molten MgCl2-KCl-NaCl for continuous immersion up to 31 days. Two electrochemical methods were used to characterize the long-term performance of the reference electrodes at 500°C in an inert atmosphere using a Mg rod as the working electrode: open circuit potential (OCP) and onset of Mg(II) reduction as measured by cyclic voltammetry. There was only a 3% difference between the average OCP measured versus the mullite and MgO reference electrodes. Potential drift was higher for mullite (-1.34 mV/day) than MgO (0.22 mV/day). But both drift rates were very low compared to the OCP data standard deviation, indicating that no significant drift was measured for us to 31 days.