The reaction of arsenic, MgSO4 and S2Cl2 in acetonitrile yielded the title compound along with 4,5- dichloro-1 ,2,3-dithiazolium chloride, [C2NS2Cl2]Cl (which was formed from the solvent and S2Cl2). The crystal strucutre of [Mg(NCCH3)6][As4Cl14] was determined by X-ray diffraction. It is triclinic, space group P1¯, Z = 2, R = 0.049. The Mg2+ ion is coordinated by six acetonitrile ligands. The anions consist of AsCl4- and AsCl3 units which are associated to chains along the a axis by As ··· Cl contacts. Each As atom has distorted octahedral coordination.