This work assesses the applicability of the EMF method for determination of the values of thermodynamic functions for the Li–S electrochemical system in lithium–sulfur cells with various states of charge. It is shown that the EMF method can be used for determination of the values of thermodynamic functions for the Li–S electrochemical systems in lithium–sulfur cells with various states of discharge only in the first charge–discharge cycle. The EMF method is inapplicable in the following charge–discharge cycles owing to disturbance of equilibrium in the electrochemical system due to the direct chemical interaction between sulfur and high-order polysulfides of lithium (Li2Sn, n > 4) and the metallic lithium electrode. The thermodynamic functions of the Li–S system with different sulfur reduction degree are in the following ranges: ΔG = –480…–410 kJ/mol; ΔH = –490…–420 kJ/mol; ΔS = –120…–20 J/(mol K) at the temperature of 303 K. Quantum–chemical calculations of the values of thermodynamic functions are carried out for electrochemical reduction of sulfur and lithium polysulfides. The calculated values of thermodynamic functions agree reasonably with the measured values. The thermodynamic efficiency of energy conversion is estimated for discharge of lithium–sulfur cells at 30°C; it is 93–98%.