Mechanisms associated with the isomerization of the 0-methylethylene oxonium ion and its tetramethyl-substituted analogue have been explored using correlated electronic structure calculations. The minima and transition states associated with inversion at the oxygen atom, as well as those associated with opening of the epoxide ring, have been characterized. The calculated barrier to inversion at the oxygen atom for the 0-methylethylene oxonium ion, 15.7 kcal/mol, agrees well with the experimentally determined value, 10 ±2 kcal/mol. Our calculations indicate that a significantly higher barrier exists for the ring-opening mechanism that leads to more thermodynamically stable structures. This work includes the first known calculations on the 0-methyl-2,3-dimethyl-2-butene oxonium ion along with transition states and intermediates associated with ring opening and inversion at the oxygen atom. Results show that there is a significantly lower barrier to ring opening as compared to the O-methylethylene oxonium ion species, leading to a lower probability of isolating this species. The effects of basis sets and correlation techniques on these ions were also analyzed in this work. Our results indicate that the B3LYP/6-31G* level is reliable for obtaining molecular geometries for both minima and transition states on the C3H7O + and C7H15O+ potential energy surfaces.