AbstractThe thermal decomposition (polymerization, depolymerization, ring interconversion) of a number of sulfur allotropes (S6, S7, S8, S10, S12, S13, S20, polymeric sulfur) has been investigated theoretically (on the basis of Gee's theory) and experimentally by differential scanning calorimetry (DSC) and high‐pressure liquid chromatography (HPLC) in the temperature region of 30–250°C. While the polymerization of liquid S8 is endothermic and endentropic (ΔS > 0), liquid S7 polymerizes exothermically and endentropically, and liquid S6 exothermically but exentropically (ΔS > 0). Therefore, a floor temperature exists for the polymerization of S8 and a hypothetical very high ceiling temperature for the polymerization of S6, while S7 is unstable with respect to polymerization over the whole temperature region. Excepting S8, all investigated cyclic sulfur allotropes yield polymeric sulfur on heating to 60–150°C followed by depolymerization to the equilibrium sulfur melt consisting mainly of S8, some S7, and traces of S6, S9, S12 and other rings.Polymeric sulfur (Sμ) slowly dissolves in CS2 at 20°C to give S8, S7 and traces of other rings (mainly S6, S9, S12). On heating of the Sμ/CS2 mixture in sealed ampoules to 80–100°C complete dissolution takes place within several hours or days and the rings S8 and S7 are the main products.