Sodium/°-alumina/phosphorus-sulfur cells utilizing P/S ratios of 0, 0.143, 0.332, and 1.17 at temperatures from 350 to 500°C were studied by measurement of the equilibrium cell voltages at open circuit, and the steady-state cell voltages at current densities up to 70 mA/cm2. States of charge, represented by sodium atom fraction in the P-S electrode, ranged from 0 to 0.4. Open-circuit voltages up to 2.65 V were measured. Theoretical specific energies up to 825 Wh/kg were calculated from the open-circuit potential vs. composition data. When phosphorus was added to the sulfur electrode in the P/S ratio 0.143, the cell resistance was two to ten times greater than that with sulfur alone.