Cinnabar use on painting is historically documented from ancient times, together with its tendency to darken. The most accepted mechanism for this kind of deterioration considers cinnabar (α-HgS) transformation into its polymorph metacinnabar, β-HgS, but recent studies have cast doubt on this hypothesis. On this background, an in situ high-temperature X-ray powder diffraction study of the thermal behaviour of cinnabar has been carried out. Data, measured in transmission geometry on a non-hermetically sealed capillary, indicate that the α-HgS (trigonal)→β-HgS (cubic) phase transition occurs at 673 K and is completed at 698 K due to kinetics reasons. The thermal expansion of cinnabar is fairly isotropic, the a-parameter being slightly softer against heating. A fast cooling of metacinnabar at room temperature (T) produced a mixture of both polymorphs. The analysis of the kinetics of the β-HgS→α-HgS transformation at room-T shows a low rate of conversion, but not low enough to be consistent with a persistence of metacinnabar for a time span of 2000 years as hypothesized in reference data for Pompeian frescoes.