Six isotopic derivatives of the complex (CH3)3N-SO3 have been studied in the gas phase by microwave spectroscopy. The N-S bond length is 1.912(20) Å, and the NSO angle is 100.1(2)°. The dipole moment, determined from Stark effect measurements, is 7.1110(69) D, representing an enhancement of 6.5 D over the sum of the dipole moments of the free monomers. Analysis of the 14N nuclear hyperfine structure indicates that about 0.6 e is transferred from the nitrogen to the SO3 upon formation of the complex. Comparison between the gas-phase structure and that previously determined for the adduct in the solid state reveals small but significant differences, indicating that the formation of the dative bond is slightly less advanced in the gas. Gas-phase and solid-state structural data are compared for several related amine - SO3 systems.