Geometries of the molecules HCN, CH3CN, NH3, SO3 and those of the complexes HCN–SO3, CH3CN–SO3 and NH3–SO3 were optimized at the RHF/6-31G∗∗ and MP2/6-31G∗∗ levels. Partial geometry optimization calculations were also carried out in each case at the RHF level for different fixed N–S distances. Hybridization displacement charges (HDC) and molecular electrostatic potential-derived CHelpG charges at the atomic sites were obtained at the RHF and MP2 levels. Dipole moments and values of charge transfer (CT) (from each of HCN, CH3CN and NH3 to SO3) obtained using HDC point charges vary with N–S distance in the same way as those obtained using CHelpG point charges at the RHF level. Further, the calculated values of CT as obtained using both the CHelpG charges and HDC decrease with increasing N–S distance. The experimental and the present calculated values of CT in NH3–SO3 at the optimized and experimental N–S distances agree quite well while the calculated values of CT in HCN–SO3 and CH3CN–SO3 help resolve an existing ambiguity in this context. The locations of the different HDC components with respect to the corresponding atoms have been obtained. The calculated results indicate that the complexes are bound largely by CT.