A series of donor-acceptor complexes containing sulfur trioxide have been studied in the gas and condensed phases using density functional theory. The condensed phase is represented using the polarizable continuum model. The systems investigated include complexes of nitrogen-containing donor molecules, (CH3)(n)H-3-N-n (n = 0 - 3), with SO3 and complexes of oxygen-containing donor molecules, (CH3)(m)H2-mO (m = 0 - 2), with SO3. Significant differences are observed between the gas- and condensed-phase properties of the complexes as a result of the ability of the condensed-phase medium to support higher charge separation between the donor and acceptor. The gas/condensed-phase behavior of two nitrogen-containing complexes, (CH3)H2N-SO3 and (CH3)(2)HN-SO3, has been investigated for the first time. These complexes exhibit properties intermediate to the previously observed H3N-SO3 and (CH3)(3)N-SO3 complexes. Systematic trends in the gas- and condensed-phase structure and properties have been observed as methyl groups are added to the donor molecule. In addition, two oxygen-containing complexes, CH3OH-SO3 and (CH3)(2)O-SO3, have been characterized for the first time. The differences between the gas- and condensed-phase properties of the oxygen-containing complexes are, in many cases, larger than those of the nitrogen-containing complexes, and therefore they represent an intriguing new class of complexes for potential experimental observation. Finally, a strong correlation between the charge transfer and binding energy has been obtained for both the nitrogen- and oxygen-containing complexes of sulfur trioxide.