The C-2-symmetry structure of oxirene has been examined using ab initio molecular orbital calculations with large basis sets and a variety of methods of including electron correlation. Different qualitative conclusions regarding the nature of oxirene are reached, depending on the choice of basis set and method of electron correlation incorporation. With certain combinations of basis set and theoretical method, the symmetric oxirene structure is found to be a saddle point on the potential energy surface, while for other combinations oxirene is a local minimum. Inclusion of triple excitations in the correlation treatment has a large effect, tending to make the curvature of the surface corresponding to a ring-opening distortion of the C-2v-symmetry structure less positive (or more negative). This is counterbalanced by a basis set effect, with inclusion of f functions making the curvature more positive. At our highest level of theory, CCSD(T) with basis sets of triple-zeta quality and including multiple d shells and an f shell on C and O and multiple p shells and a d shell on H, oxirene is a genuine minimum under harmonic vibrational analysis, with a ring-opening frequency of 139-163 cm(-1)