The characterization of the heterocyclic radical 1,2,5-thiadiazolo[3,4-b]-1,3,2-dithiazolo[3,4-b]pyrazin-2-yl (TDP-DTA) is described. The compound is prepared by treatment of 5,6-dithiolo-1,2,5-thiadiazolo[3,4-b]pyrazine with S3N3Cl3 and purified by fractional sublimation in vacuo. The results of cyclic voltammetry and ESR analysis of TDP-DTA and related heterocyclic dithiazolyls indicate that the spin distributions and donor/acceptor properties of these radicals are extremely sensitive to the nature of the 4,5-substituents. The crystal structure of TDP-DTA has been determined at two temperatures. At 293 K, the crystals are triclinic, space group P (1) over bar, with a = 4.4456(8), b = 8.407(2), c = 9.671(3) Angstrom, alpha = 71.34(2), beta = 89.28(2), gamma = 87.80(2)degrees, Z = 2 (for C4N5S3); at 150 K the crystals are triclinic, space group P (1) over bar, with a = 7.489(7), b = 9.593(4), c = 10.759(6) Angstrom, alpha = 65.77(4), beta = 74.10(6), gamma = 74.64(6)degrees, Z = 2 (for C8N10S6). At 293 K the structure consists of ribbons of TDP-DTA radicals packed in a slipped pi-stack arrangement. In the low temperature phase alternate layers of ribbons are shifted laterally to produce arrays of dimers. Within these dimers three long (3.401(5), 3.460(5) and 3.511(5) Angstrom) interannular S-S contacts link the two molecules. A multiple "tectonic plate" slippage mechanism is proposed to account for the interconversion of the two phases. The structural results are discussed in the light of variable temperature magnetic susceptibility measurements.