Abstract Reaction of py · PS2Cl (py = pyridine) with benzyl amine in the presence of NEt3 yields triethyl-ammonium 1,3-dibenzyl-2,4-dithioxo-1,3-diaza-2λ5,4λ5-diphosphetidine-2,4-dithiolate which reacts with MeI to give the corresponding methyl ester. By fractional crystallization from cyclohexane three modifications of the methyl ester were obtained (two trans-(A and A') and one cis-modification (B)). The crystal structure of these modifications have been determined by single crystal X-ray diffraction. Crystal data: A: triclinic, space group P1̄ , Z = 2, a = 7.100(3), b = 10.088(6), c = 17.785(6) Å , α = 99.18(4)°, β = 93.75(5)°, γ = 106.96(4)°, A': monoclinic, P21/c, Z = 2, a = 7.933(3), b = 13.689(3), c = 9.917(4) Å, β = 111.98(3)°; B: orthorhombic, P cab, Z = 8, a = 12.962(6), b = 16.581(6), c = 19.475(7).