The composition of a complex equilibrium mixture formed upon dissolution of (Se6I2)[AsF6](2)center dot 2SO(2) in SO2(l) was studied by Se-77 NMR spectroscopy at -70 degrees C with both natural-abundance and enriched Se-77-isotope samples (enrichment 92%). Both the natural-abundance and enriched NMR spectra showed the presence of previously known cations 1,4-Se6I22+, SeI3+, 1,1,4,4-Se4I42+ Se-10(2+), Se-8(2+), and Se-4(2+). The structure and bonding in 1,4-Se6I22+ and 1,1,4,4-Se4I42+ were explored using DFT calculations. It was shown that the observed Se-Se bond alternation and presence of thermodynamically stable 4p pi-4p pi Se-Se and 4p pi-5p pi Se-I bonds arise from positive charge delocalization from the formally positively charged tricoordinate Se+. The Se-77 chemical shifts for cations were calculated using the relativistic zeroth-order regular approximation (ZORA). In addition, calculations adding a small number of explicit solvent molecules and an implicit conductor-like screening model were conducted to include the effect that solvent has on the chemical shifts. The calculations yielded reasonable agreement with experimental chemical shifts, and inclusion of solvent effects was shown to improve the agreement over vacuum values. The Se-77 NMR spectrum of the equilibrium solution showed 22 additional resonances. These were assigned on the basis of Se-77-Se-77 correlation spectroscopy, selective irradiation experiments, and spectral simulation. By combining this information with the trends in the chemical shifts, with iodine, selenium, and charge balances, as well as with ZORA chemical shift predictions, these resonances were assigned to acyclic 1,1,2-Se2I3+, 1,1,6,6-Se6I42+, and 1,1,6-Se6I3+, as well as to cyclic Se7I+ and (4-Se7I)(2)I3+. A preliminary natural-abundance Se-77 NMR study of the soluble products of the reaction of (Se-4)[AsF6](2) and bromine in liquid SO2 included resonances attributable to 1,1,4,4-Se4Br42+. These assignments are supported by the agreement of the observed and calculated 77Se chemical shifts. Resonances attributable to cyclic Se7Br+ were also observed. The thermal stability of (Se6I2)[AsF6](2)center dot 2SO(2)(S) was consistent with estimates of thermodynamic values obtained using volume-based thermodynamics (VBT) and the first application of the thermodynamic solvate difference rule for nonaqueous solvates. (Se6I2)[AsF6](2)center dot 2SO(2)(s) is the first example of a SO2 solvate for which the nonsolvated parent salt, (Se6I2)[AsF6](2)(s), is not thermodynamically stable, disproportionating to Se4I4(AsF6)(2)(s) and Se-8(AsF6)(2)(S) (Delta G degrees for the disproportion reaction is estimated to be -17 +/- 15 kJ mol(-1) at 298 K from VBT theory).