Crystals of the title compound (commonly called thiodithiazyl chloride) are orthorhombic, space group Pbca, with a= 7.88(1), b= 14.46(1), c= 8.19(1)Å, and Z= 4. The structure was refined to R= 0.031 using 563 (of the 792 unique) measured intensities for which l > 3σ(l). The dimeric cation contains two centrosymmetric planar S3N2 rings in a chair (trans) conformation. Distances are S(1)–N(1) 1.563(5), N(1)–S(3) 1.618(4), S(2)–S(3) 2.131(2), S(2)–S(3I) 2.916(2), and Cl–S(2,3) 2.842(2), 2.858(2)Å. The dihedral angle between the S3N2 ring plane and the central S4 ring is 110.4(3)°. Crystals of S6N4Cl2 were grown by three different methods based on (i) a reversible temperature-dependent ring expansion–contraction (S6N4Cl2↔ S4N3Cl) equilibrium in SO2–SOCl2 mixtures, (ii) a complex chemical transport (vapour–liquid–solid) in a sealed tube containing (initially) S3N2Cl2 and liquid SO2, and (iii) a controlled slow reaction of S3N2Cl2 with liquid SO2. The last method is a particularly convenient laboratory synthesis of pure S6N4Cl2. Solid S6N4Cl2 shows a strong e.s.r. signal which is independent of temperature (in the range –139 to +22 °C), indicating the presence of a little monomer.