Thermal analysis and X-ray powder diffraction data have been used to show that realgar synthesized directly from arsenic and sulfur exists in the high temperature β phase, which is metastable at room temperature. The X-ray powder data indicated that the metastable β phase is orthorhombic with lattice constants a = 9·95 A ̊, b = 13·49 A ̊ and c = 7·07 A ̊. The enthalpy of the α to β transition and the enthalpy of fusion were measured as 0·83 kcal/mole and 6·0 kcal/mole respectively. The observed stability of β realgar relative to decomposition to As2S3 was greater than that reported in the literature for α realgar. © 1970.