Relative energies of several C2H6OS molecules and C2H7OS+ ions were obtained by ab initio and density functional theory calculations. Geometry optimizations with Becke＇s hybrid functional (B3LYP) and the 6-31+G(d,p) basis set gave good-quality equilibrium structures which did not improve significantly by B3LYP calculations using the larger 6-311+G(2df,2p) basis set. Among C2H6OS isomers with S-O bonds, dimethyl sulfoxide (DMSO, 1) was the most stable species, followed by methyl methanesulfenate (3), S-hydroxy-S-methyl-S-methylenesulfurane (DMSO enol, 2), S-methoxy-S-methylene-S-(H)sulfurane (4), and (S,S-H-2)2-oxa-1-thietane (5). O-protonated dimethyl sulfoxide (1aH(+)) was the most stable C2H7OS+ isomer of those studied. Ion 1aH(+) was separated from the less stable isomers by substantial potential energy barriers. The G2(MP2) proton affinity of DMSO (885 kJ mol(-1)) was in excellent agreement with the value from equilibrium measurements. DMSO-enol (2) was predicted to be a strong gas-phase base for protonation at the methylene and hydroxyl groups. S-protonation in 3 was 44 kJ mol(-1) more exothermic than O-protonation. In general, proton affinities were overestimated by B3LYP calculations and underestimated by MP2 calculations with all basis sets used. Empirical averaging of the B3LYP and MP2 values, obtained from calculations with the 6-311+G(2df,2p) basis set, provided an improved agreement with experimental or accurate G2(MP2) proton affinities.