The photoionization efficiency (PIE) spectrum of HSO was measured in the spectral range (107-130) nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as the radiation source. HSO radicals were generated by reacting O atoms with various organothiol compounds, C2H5SH, 2-C3H7SH, or HSC2H4SH, in the flow tube. The ionization energy of HSO was determined for the first time and found to be (9.918 +/- 0.016) eV. GAUSSIAN-2 calculations predict 9.897 eV for ionization to HSO+, the singlet ground state of the molecular ion, in satisfactory agreement with the experimental result. The onset to tripler HSO+ may occur at (11.15 +/- 0.04) eV. A vibrational frequency of HSO+ of (1150 +/- 160) cm(-1) was derived from the separation of steps in the PIE spectrum. The heat of formation of HSQ(+) was also derived and calculated to be Delta(f)H(298)degrees(HSO+) = (228 +/- 5) kcal mol(-1).