In order to optimize the cycling performance of lithium–sulfur (Li–S) batteries, it is highly important to understand the structural and morphological changes of the sulfur cathode under operating conditions. In this work, a suitable cell for in-situ X-ray diffraction (XRD) analysis was designed to study the structural modifications on the cathode of Li–S batteries during electrochemical cycling. As a result, the formation of reaction products during charging and discharging was monitored in operando. The formation of dilithium sulfide (Li2S) and the recrystallization of sulfur during the first cycle could be identified. Moreover, these crystalline phases were semi-quantitatively determined by means of XRD analysis. Morphology changes of the cathode due to electrochemical cycling were characterized using scanning electron microscopy (SEM).