The SO3-NH3-X-n(n) (n = 0-3; X = F, Cl) acceptor-donor complexes have been investigated at the G2MP2 level of theory. The G2MP2 results show that the successive fluorine and chlorine substitution on the nitrogen decreases the stability of SO3-NH3-nFn and SO3-NH3-nCln complexes, in accord with the decreasing basicity of the NH3-nFn and NH3-nCln ligands. The natural bond orbital (NBO) partitioning schemes show that the shortening of the N-F and N-Cl bond lengths, upon complexation, is due to an increasing "s" character in these bonds.