The photoionization efficiency (PIE) spectrum of C2H5SO was measured in the spectral range 110-150 nm by means of a photoionization mass spectrometer coupled to a synchrotron radiation source. C2H5SO radicals were generated by reacting O atoms with C2H5SC2H5 or C2H5SSC2H5 in a discharge Bow tube. The ionization energy of C2H5SO was determined to be (8.71+/-0.02) eV, in agreement with theoretical calculations of 8.8 eV for ionization from C2H5SO to singlet C2H5SO+ using the G2 method, PIE spectrum also shows a sharp increase at 10.0 eV, corresponding to the ionization energy from C2H5SO to triplet C2H5SO+ predicted by theory. Theoretical calculations also predict two stable conformers with sim;lar energies for each of C2H5SO, singlet C2H5SO+, and tripler C2H5SO+, respectively; these conformers are indistinguishable in our experiments. (C) 1997 American Institute of Physics.