The state assignments corresponding to the second and third ionization potentials of oxirane have been the subject of some uncertainty due to the great sensitivity of Koopmans' theorem predictions to the type of calculation performed for this molecule. In this study, the assignments made by Basch et al. for oxirane are confirmed through measurement of the photoelectron spectra of methyl-, 1,1-dimethyl-, 1,2-dimethyl, ethyl-, vinyl-, phenyl-, chloromethyl-, and fluoromethyloxirane. The second and third ionizations of thiirane are confirmed to be of the opposite order from those in oxirane by correlations of these values with those reported here for methyl-, vinyl, and methoxymethylthiirane.In these compounds, mono-substitution by alkyl groups causes a decrease in ionization potential not obviously related in a simple way to the type of orbital involved. Thus, assignments cannot be made straightforwardly by observing ionization potential changes caused by hyper-conjugating or inductive substituents. Alkyl group substitution in both oxirane and thiirane lowers IP's in the same order: a2 > b2 > a1 > b1. The effects of alkyl and heteroalkyl groups on the four lowest IP's of oxirane are linearly related to the electronegativities of the groups, and to the influence of alkyl and heteroalkyl substituents on the π IP of ethylene.Comparisons of the IP's of ethylheteranes and dimethylheteranes are quite useful in determining the site of localization of orbital density in the various orbitals.Conjugating substituents, which for symmetry reasons, selectively interact with only a few orbitals make definite assignments possible. Comparisons of experimental ionization potential changes and those predicted by Koopmans' theorem using ab initio STO-3G calculations are in good agreement.