The rearrangement of dihydronoracronycine (3) to dihydroisonoracronycine (4) proceeds by way of an intermolecular reaction rather than an intramolecular reaction as had originally been supposed. Deuterium-labeling studies showed the incorporation of deuterium at C3, C12, and the geminal methyl positions of dihydroisonoracronycine (4). Because no reaction occurred when the bisnor derivative was treated in the same manner as 3, it appears that the chromene geminal methyl groups are important for the rearrangement to occur.