An in situ high-temperature X-ray powder diffraction study of the thermal behavior of realgar (α-As4S4) has been carried out. Data, measured in transmission geometry on a non-hermetically sealed capillary, indicate that the realgar → β-As4S4 phase transition starts at 558 K and is completed at 573 K due to kinetics. Melting starts at 578 K and is completed at 588 K. Thermal expansion of realgar is significant and fairly isotropic. In fact, the a-and b-parameters expand almost at the same rate, whereas the c-parameter is slightly softer against heating. Moreover, the β-angle contracts as temperature is raised. The geometry of the As4S4 molecule is largely independent from heating. The lengthening of a few As-S and As-As contacts above or near the sum of the As,S van der Waals radii represents the driving force of the phase transition. In addition, the thermal behavior of arsenolite As2O3 and non-stoichiometric As8S8+x crystals produced from As4S4 melt recrystallization has been investigated. Two members located along the β-As4S4-alacranite (As8S9) series joint were identified at RT: a term close to the β-As4S4 end-member (As8S8+x: x = ca. 0.1) and one term of approximate As8S8.3 composition. The thermal expansion of β-As4S4 is significantly anisotropic following the αb > αa > αc relationship. This is clearly the result of the different packing scheme of the As4S4 cages in β-As4S4 with respect to realgar. The dependence of cell parameters and volume of As8S8.3 is more complicated. In fact, a strong discontinuity on the dependence of cell parameters and volume is observed in the 403–443 K thermal range, i.e., that at which As8S8.3 converts partly to realgar. A significant volume expansion is observed as a result of a change of composition to As8S8.7.