Appearance potentials and relative abundances are reported for the principal positive ions in the mass spectra of ethylene oxide and propylene oxide, obtained using a Bendix 12-100 time-of-flight mass spectrometer. Probably ionization and dissociation processes are given consistent with computed energetics and the heats of formation of the various ions derived from the energetics are tabulated. Molecular ionization potentials are calculated using the equivalent orbital method of Hall are compared to the observed ionization potentials of 10.65 plus/minus 0.1 e.v. for ethylene oxide and 9.80 plus/minus 0.1 e.v. for propylene oxide.