[en] The ionization energy of SO3 is determined to be 12.828±0.010 eV from photoionization mass spectrometric measurements of SO3. Two autoionization Rydberg series converging to the SO3(2E') state (17.901 eV) are identified. Three window resonances observed in the region 610--690 A are assigned as members of an autoionizing Rydberg series converging to the SO+3(2A1) state (20.603 eV)