The structure and dipole moment of HCN ... HCN-SO3 have been determined by rotational spectroscopy. The N-S bond distance is 2.470(20) Angstrom, which is 0.107(21) Angstrom shorter than that in HCN-SO3. In contrast, the N ... H distance, 2.213(29) Angstrom, is the same to within experimental uncertainty as that in (HCN)(2). The dipole moment of (HCN)-N-15 ... (HCN)-N-15-(SO3)-S-32 is 8.640(19) D, representing an enhancement of 1.238(19) D over the sum of the dipole moments of HCN and HCN-SO3. The results indicate significant changes in the HCN-SO3 subunit upon interaction with a single HCN "solvent" molecule, with relatively little change in the HCN-HCN interaction. The hypersensitivity of the HCN-SO3 moiety to the presence of an additional HCN arises because the dative bond is partially formed, and we suggest that partially bound systems may offer sensitive probes of microsolvation.