The donor-acceptor complex C5H5N-SO3 has been studied in the gas phase using rotational spectroscopy. The adduct has the expected geometry in which the nitrogen lone pair is directed along the C-3 axis of the SO3 and the SO3 undergoes free or nearly free internal rotation within the complex. The N-S bond length is 1.915(1) Angstrom, and the NSO angle is 98.91(2)degrees, indicating that the formation of the dative bond is nearly, but not entirely, complete. Small but significant changes in the heavy-atom ring structure of the pyridine upon complexation are also measured by a series of C-13 substitution experiments. Analysis of the N-14 quadrupole coupling constants indicates a transfer of approximately 0.54 electrons away from the pyridine upon formation of the dative bond. In the series of complexes of SO3 with HCN, CH3CN, H3N, C5H5N, and (CH3)(3)N, electron-transfer values increase as the bond length decreases, and these changes are shown to accompany a gradual decrease in the energy gap between the lone pair orbital of the base and the LUMO of SO3. Tabulated values of hardness (eta) and electronegativity (chi) give rough but irregular estimates of these energy gaps, and the best correlations are obtained with energies derived directly from ionization potentials for the pertinent orbitals. Binding energies (D,) have also been determined for the following complexes at the MP2/aug-cc-pVTZ level of theory/basis set: (CH3)(3)N-SO3 (36.3 kcal/mol), C5H5N-SO3 (25.5 kcal/mol), H3N-SO3 (19.6 kcal/mol), CH3CN-SO3 (9.0 kcal/mol), HCCCN-SO3 (7.4 kcal/mol), and HCN-SO3 (7.3 kcal/mol). These values also correlate with the energy gap between the donor orbital of the base and the acceptor orbital of the SO3.