Work at Brookhaven National Laboratory on the electrochemically regenerative hydrogen--chlorine energy storage system has included electrochemical investigations, materials studies, and technoeconomic assessment. Electrochemical studies have confirmed the reversibility of the cell reactions and the possibility of using the same cell in the electrolysis and fuel cell mode. The hydrogen--chlorine cell differs from most batteries in that the open circuit potential varies appreciably with temperature and depth of discharge. The temperature variation of the open circuit potential reflects the large negative entropy of formation of HCl. A detailed heat and mass balance analysis has been carried out for the H/sub 2//Cl/sub 2/ system for one method of reactant storage and two schemes of heat exchange between the hydrochloric acid storage subsystem and the reactant storage subsystems. Characterization of Nafion membranes in H/sub 2//Cl/sub 2/ cells is reported. From a cost comparison on a 20 MW/200 MWh electrochemically regenerative hydrogen--halogen system it was concluded that the use of either clorine or bromine or alternative methods of chlorine storage had an insignificant effect on the overall cost of the system. The most cost effective method of hydrogen storage is very dependent on the cost of activated metal hydrides.