The spectral dependence of the photoionization efficiency of HSSH was obtained over the wavelength range 110-140 nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as the radiation source. The HSSH was generated in the flow tube at room temperature via the sequential reactions : O + H2S --> OH + HS; HS + HS (+ M) --> HSSH (+ M), M being any third body. The ionization energy of HSSH was determined to be 9.40 +/- 0.02 eV. This result agrees satisfactorily with the reported value measured from photoelectron spectra. The heat of formation of the ion HSSH+, Delta(f)H(298)(0)(HSSH+), was calculated to be 220.6 +/- 2 kcal mol(-1). The generation of HSSH from the self-reaction of HS radicals might indicate a previously neglected role in the atmospheric sulfur cycle.