We have carried out a number of experiments to characterize some electrochemical properties of a Na/S cell. They were: measurements of the cell EMF, cell voltage and current on charge and discharge, both at constant current and with stepped currents; cell impedance from 0.5 mHz to 10 kHz; and measurements of the relaxation of the open‐circuit voltage. At the onset of the single‐phase region in the sulfur electrode, the cell impedance from 5 to 0.5 mHz rises to a maximum due to the onset of a diffusion controlled process. The measurements of cell voltage and current confirm this experimental result.