The spectrum of photoionization efficiency (PIE) of S2O2 was measured in the spectral range 105-130 nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as a radiation source. S2O2 was generated from self reaction of SO radicals, SO+SO-->S2O2, in a gaseous flow system. SO radicals were produced on reacting O atoms with CS2 and OCS, The ionization energy (IE) of S2O2 was determined to be (9.93+/-0.02) eV.