The gas-phase structures of HCN-SO3 and CH3CN-SO3 have been determined by microwave spectroscopy in a supersonic jet. Both adducts are symmetric tops with the nitrogen bonded to the SO3. In HCN-SO3, the N-S distance is 2.577(6) Angstrom and the NSO angle is 91.8(4)degrees. In CH3CN-SO3, the bond length and bond angle are 2.466(16) Angstrom and 92.0(7)degrees, respectively. The N-S distances are significantly shorter than the sum of van der Waals radii, and the structures are indicative of N-S dative bonds which are in their early stages of development. N-14 nuclear hyperfine structure is consistent with this assessment. The bond length-bond angle relationship for a series of SO3 complexes with amine and nitrile donors is examined and compared with that for a similar series of complexes of BF3 and BH3. The variation of bond angle with bond length is strikingly similar for both sets of systems despite the differing atomic sizes of boron and sulfur. With a given base, however, the degree of bond formation to SO3 appears to lag that to BF3.