A topological analysis on methyl substituted oxiranes and their protonated forms has been carried out on B3LYP/ 6-311++G(d,p) electron densities. The oxygen atom bears in oxiranes a large negative charge, though lower than that displayed in open chain compounds, in line with the depletion of oxygen electronegativity predicted by the hybridisation model. Oxygen protonation of several oxiranes with methyl substituents results in ring opening through a bifurcation catastrophe. The theoretical protonation energy, ZPVE corrected, correlates well with the AIM-calculated charge on the attached proton, which lends support to the consideration of protonation as a charge transfer process. (C) 2003 Elsevier Science B.V. All rights reserved.