Calculations at the MP2/6-31G*//MP2/6-31G* level show there are concerted asynchronous pathways connecting protonated propene oxide and protonated propanal. With cleavage of the C-O bond of protonated propene oxide, the preference for rotation of oxygen away from the more hindered face of the oxirane plane containing the methyl group is quantified as 2 kcal/mol. This pathway involves two distinct steps; first, rupture of the oxirane and, second, hydride migration. The latter does not commence until rupture of the C-O bond is complete. The combination of these two steps defines a concerted asynchronous rearrangement pathway. The reaction is predicted to show a 20:1 preference for migration of the proton trans to the methyl over the cis.