The energetics of dissocn. reactions of S82+ into stoichiometric mixts. of Sn+, n = 2-7, and Sm2+, m = 3, 4, 6, 10, were investigated by the B3PW91 method [6-311+G(3df)//6-311+G*] in the gas phase and in soln., with solvation energies calcd. using the SCIPCM model and in some cases also the COSMO model [B3PW91/6-311+G*, dielec. consts. 2-30, 83, 110]. UV-vis spectra of all species were calcd. at the CIS/6-311G(2df) level and for S42+ and S62+ also at the TD-DFT level (BP86/SV(P)). Std. enthalpies of formation at 298 K were derived for S32+ (2538 kJ/mol), S62+ (2238 kJ/mol), and S102+ (2146 kJ/mol). A comparison of the obsd. and calcd. UV-vis spectra based on our calcd. thermochem. data in soln. suggests that, in the absence of traces of facilitating agent (such as dibromine Br2), S82+ dissocs. in dil. SO2 soln. giving an equil. mixt. of ca. 0.5S62+ and S5+ (K ~ 8.0) while in the more polar HSO3F some S82+ remains (K ~ 0.4). According to our calcns., the blue color of this soln. is likely due to the p*-p* transition of the previously unknown 10p S62+ dication, and the previously assigned S5+ is a less important contributor. Although not strictly planar, S62+ may be viewed as a 10p electron Huckel-arom. ring contg. a thermodynamically stable 3pp-3pp bond [d(S-S) = 2.028 .ANG.; t(S-S-S-S) = 47.6 Deg]. The computations imply that the new radical cation S4+ may be present in sulfur dioxide solns. given on reaction of sulfur oxidized by AsF5 in the presence of a facilitating agent. The std. enthalpy of formation of S6(AsF6)2(s) was estd. as -3103 kJ/mol, and the disproportionation enthalpy of 2S6(AsF6)2(s) to S8(AsF6)2(s) and S4(AsF6)2(s) as exothermic by 6-17 kJ/mol. The final preference of the obsd. disproportionation products is due to the inclusion of solvent mols., e.g., AsF3, that addnl. favors the disproportionation of 2S6(AsF6)2(s) into S8(AsF6)2(s) and S4(AsF6)2(AsF3)(s) by 144 kJ/mol. [on SciFinder (R)]