The oxirene system has received much attention recently; most theoretical studies have concentrated on the unsubstituted molecule while most experimentation has involved substituted oxirenes. In an effort to bridge the information gap between theory and experiment we have undertaken an ab initio study of substituted oxirenes. Predictions at various levels of theory up to the double-zeta plus polarization single and double coupled cluster with perturbative triples correction [DZP CCSD(T)] are presented for the series of substituted oxirenes with the formula X(2)C(2)O, where X = BH2, CH3, NH2, OH, and F. Predictions from this level of theory are thought to be quite dependable on the basis of earlier work. Stationary points are compared among themselves and with the parent oxirene (X = H) and the molecular orbitals involved in ring bonding are analyzed, providing insight into the nature of the chemical bonding. Only the dimethyloxirene molecule (in addition to the earlier reported unsubstituted oxirene) is predicted to be a minimum on the DZP CCSD(T) surface. All other substituted oxirenes in the series are predicted to be transition states at that or lower levels of theory. Preliminary data from a benzooxirene study are also included.