Rotational spectra of HCN-SO 3-Ar and HCN-SO 3-CO, and several of their isotopically substituted derivatives, have been observed by Fourier transform microwave spectroscopy. Both complexes are symmetric tops with the Ar or CO weakly bound to the SO 3 on the side opposite the HCN. The N-S and S-Ar distances in HCN-SO 3-Ar are 2.5905(19) and 3.4465(50)Å, respectively. In HCN-SO 3-CO, the N-S and S-C distances are 2.6563(14) and 3.0109(48)Å, respectively, representing a marked increase relative to the analogous distances in HCN-SO 3 and SO 3-CO. These results are in sharp contrast to previous work on HCN-HCN-SO 3, in which the second HCN attaches to the HCN side of the HCN-SO 3 and causes a large contraction of the partially formed N-S bond. Stark effect measurements have been performed and the resulting dipole moments are 4.230(10) D for HC 15N-SO 3-Ar and 3.678(11) D for HC 15N-SO 3-CO. These values are very near the differences between the dipole moments of the constituent dimers, suggesting that the dipole moments of the trimers are the vector sums of the dimer moments. Ab initio calculations for both complexes in the HCN-SO 3-X and X-HCN-SO 3 configurations are reported.