In comparison with the known orthorhombic polymorph oftrans-2,4-di-tert-butyl-2,4-dithioxo-1,3-dithia-2,4-diphosphetane, C(8)H(18)P(2)S(4), (I) [Shore, Pennington, Noble & Cordes (1988). Phosphorous Sulfur, 39, 153-157], the new crystallographic modification is monoclinic and the corresponding solid density is markedly higher. In both structures, the molecules have 2/m symmetry imposed by space-group symmetry and all corresponding bond lengths and angles are equal within the limits of errors.