Light-induced degradation in realgar (arsenic sulfide) has been studied by means of four-circle single-crystal X-ray diffraction and X-ray photoelectron spectroscopy. Because of the alteration of realgar exposed to light, the a lattice parameter and c sinβ value increase linearly from 9.327 to 9.385 Å and from 6.320 to 6.364 Å, respectively. In contrast, the b lattice parameter remains substantially constant. Anisotropic variations of the lattice parameters engender a continuous increase of the unit-cell volume from 799.5 to 810.4 Å3. Nevertheless, no correlation exists between the continuous increase of the unit-cell volume and the bond distance variations in As4S4 molecules because the As4S4 molecule in the unit cell expands very little during light exposure. The most pronounced change was in the distance between centroids in As4S4 cages. The spread of As4S4 intermolecular distances increases continuously from 5.642 to 5.665 Å, which directly affects the unit-cell volume expansion of realgar. In addition, the O1s peak increases rapidly after light exposure. The result substantiates the following reaction proposed by Bindi et al. (2003): 5As4S4 + 3O2 → 4As4S5 + 2As2O3. That is, realgar is transformed into pararealgar if oxygen exists and produces the As4S5 molecule. The additional S atom contributes to anisotropic expansion for the a and c axes because the direction of the additional S atom points toward [41̅4] in the unit cell. Furthermore, an S atom in the As4S5 molecule is released from one of the equivalent As-S-As linkages in As4S5 which becomes the As4S4 molecular of pararealgar. After the As4S5 molecule is divided into an S atom (radical) and the As4S4 (pararealgar type) molecule, the free S atom is re-attached to another As4S4 (realgar type) molecule, and reproduces an As4S5 molecule. The reproduced As4S5 molecule is again divided into an S atom (radical) and an As4S4 (pararealgar type) molecule. This cycle whereby realgar is indirectly transformed into pararealgar via the As4S5 molecule is promoted by light and repeated during light exposure.