Author Institution: Harvard-Smithsonian Center for Astrophysics, 60 Garden St.; Cambridge, MA 02138, and School of Engineering \& Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138; Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, 53121 Bonn, GermanyBy means of Fabry-Perot FT microwave spectroscopy, the rotational spectrum of protonated sulfur dioxide in two distinct isomeric forms, a \textit{cis} and a \textit{trans} geometry, is reported. Searches for both isomers were based on theoretical structures obtained at the CCSD(T)/cc-pwCVQZ level of theory corrected for zero-point vibrational effects at the CCSD(T)/cc-pV(T+d)Z level. At a similarly high level of theory, the \textit{cis} isomer is calculated to be the global minimum on the potential energy surface, but the \textit{trans} isomer is predicted to lie only a few kcal/mol higher in energy. The identification of \textit{cis} HOSO+ has been confirmed by detection of rotational lines of DOSO+ and HO34SO+ at precisely the expected frequency shifts. Because sulfur dioxide is a well known interstellar and extragalactic species, and because it possesses a large proton affinity, HOSO+ is a excellent candidate for radioastronomical detection
