The crystal structure of the title compound has been determined by three-dimensional single-crystal X-ray study from photographic data. Crystals are monoclinic, space group P21/m, with Z= 2 in a unit cell of dimensions a= 7·98, b= 8·10, c= 7·14 (all ±0·02)Å, β= 101·0 ± 0·5°. The structure is built of discrete As4S5 molecules of approximate C2v symmetry with As–As 2·55 Å, and mean As–S 2·24 Å. The structure was solved by direct methods and least-squares refinement gave R 0·11 for 613 observed reflections.