The rotational spectra of the hydrogen-bonded heterodimers SO 2⋯HF and SO 2⋯HCl in their vibrational ground states have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. Rotational constants A 0, B 0 and C 0, centrifugal distortion constants Δ j , Δ jk and δ j , and Cl-nuclear coupling constants X aa and X bb have been determined. The spectroscopic constants are interpreted in terms of a geometry in which the subunits are coplanar and in which the HX molecule forms a hydrogen bond to one of the oxygen atoms of SO 2. The observed geometries are compared with those predicted from electrostatic models of the intermolecular interaction.