Lithium/sulfur (Li/S) batteries have been under intense study over the last ten years, due to the growing interest in high gravimetric energy storage systems. In this work, isothermal micro-calorimetry technique was applied to pouch and cylindrical Li/S cells, in the aim to better understand the cycling mechanism of Li/S system and corresponding degradation phenomena. Correlation of the measured heat generation flow with the electrochemical and chemical processes occurring during both discharge and charge of Li/S cells has been carried out, at different C rates and ageing conditions. This work allowed to confirm the relevancy of operando calorimetry measurements for diagnostic purposes of the Li/S system.