The photoionization and dissociative photoionizations of ethylene oxide have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions C2H4O+, C2H3O+, C2H2O+, C2HO+, CH3O+, CH2O+, CHO+, C2H4+, C2H3+, C2H2+, C2H+, CH4+, CH3+, CH2+, CH+, and C+ have been obtained. In addition, the energetics of the dissociative photoionizations have been examined by ab initio Gaussian-2 calculations. The computational results are useful in establishing the dissociation channels near the ionization thresholds. The dissociation channels established may be broadly divided into two types: simple bond cleavage reactions and those involving transition structures and activation energies.