The cell chemistry of sodium/sulfur cells operating at room temperature (RT-Na/S cells) is being studied electrochemically and structurally. We show by means of X-ray photoelectron spectroscopy that the cell reaction is incomplete but prove that the end members of the cell reaction (S and Na2S) form among the expected polysulfide species Na2S x . The sulfur utilization can be improved by employing a solid electrolyte membrane (beta″-alumina) that prevents the diffusion of the soluble polysulfide species toward the sodium side. As an important finding, the Na+ conduction within the solid electrolyte phase and across the two liquid/solid interfaces results in only small overpotentials. Nevertheless the utilization of sulfur in the present RT-Na/S (475 mAh g−1) cells is lower than the theoretical value (1675 mAh g−1). One probable reason is the chemical instability of the widely used PVDF binder. Also, the thermodynamic properties of RT-Na/S cells operating at room temperature are discussed and compared with the currently much more studied RT-Li/S cells.