The 1,3,2,4-dithiadiazolyl RCNSNS(.) radicals undergo an unprecedented concerted rearrangement to the thermodynamically more stable 1,2,3,5-dithiadiazolyl RCNSSN(.) radicals by the net exchange of adjacent cyclic sulfur and nitrogen atoms. The UV-visible spectra of RCNSNS(.) (R = Ph, p-O2NC6H4, 3,5-(O2N)(2)C6H3, CF3) in solution show bands at 250 nm (strong) and 680 nm (very weak) attributable to monomer and two dimer bands at 376 and 480 nn, the positions of which are independent of the substituents, providing direct identification of the radical dimers in solution. The dimerization equilibrium constant (K-298 similar or equal to 0.7 for R = Ph) at room temperature was derived from the enthalpy and entropy changes for the dimerization of PhCNSNS(.) (Delta H-d(o) = -19.0 kJ/mol, Delta S-d degrees = -66.5 J/mol) estimated by a variable-temperature ESR spectroscopic study. In addition, RCNSNS(.) (R Bu(t), Ph) undergo an apparent unimolecular photolysis to RCN and possibly SNS. (analogue of ONO.). The photochemical rearrangement and dissociation (for R = Ph and 3,5-(O2N)(2)C6H3) were shown to proceed by irradiation of the radical dimer (376 and 480 nm) and monomer (250 nm), respectively. Thus, the radical rearrangement reasonably occurs via a concerted dimeric pathway shown by molecular orbital calculations (CNDO) to be photochemically symmetry-allowed. In addition, we propose that the radical dissociation proceeds via a concerted unimolecular photochemically symmetry-allowed process.