The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The alpha-phase crystallizes in the tetragonal space group P $(4) over bar $2(1)m and consists of pi-stacked radicals, tightly clustered about (4) over bar points and running parallel to c. The beta-phase belongs to the monoclinic space group P2(1)/c and, at ambient temperature and pressure, is composed of pi-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S center dot center dot center dot S-S center dot center dot center dot S sigma-bonds. Variable-temperature magnetic susceptibility chi measurements confirm that alpha-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in chi can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S = 1/2 centers. The dimeric phase beta-1a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (T up arrow= 380 K, T down arrow = 375 K) increase in chi and chi T. Powder X-ray diffraction analysis of beta-1a at 393 K has established that the phase transition corresponds to a dimer-to-radical conversion in which the hypervalent S center dot center dot center dot S-S center dot center dot center dot S sigma-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that alpha-1a behaves as a Mott insulator, but the ambient-temperature conductivity sigma(RT) increases from near 10(-7) S cm(-1) at 0.5 GPa to near 10(-4) S cm(-1) at 5 GPa. The value of sigma(RT) for beta-1a (near 10(-4) S cm(-1) at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and sigma(RT) increases in a manner which parallels the behavior of alpha-1a. These changes in conductivity of beta-1a are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of beta-1a confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S center dot center dot center dot S-S center dot center dot center dot S sigma-bond in beta-1a to heat and pressure is compared to that of related dimers possessing S center dot center dot center dot Se-Se center dot center dot center dot S sigma-bonds.