The isomerizations of dihydrobenzofuran and isodihydrobenzofuran were studied by the Becke three-parameter hybrid method with Lee-Yang-Parr correlation functional approximation (B3LYP). Structure and frequency calculations were carried out with the Dunning correlation-consistent polarized double xi (cc-pVDZ) and augmented aug-cc-pVDZ basis sets. The energetics was calculated using coupled cluster theory CCSD(T). Both reactions proceed via stepwise mechanisms. The potential energy surface of the dihydrobenzofuran --> o-hydroxystyrene isomerization has one intermediate and two transition states. In the isodihydrobenzofuran --> o-tolualdehyde isomerization, there are two intermediates and three transition states on the surface. Whereas a stable intermediate is produced in the dihydrobenzofuran isomerization, the intermediates in isodihydrobenzofuran are unstable biradicals. The last step in both isomerizations is a H-atom migration. The intermediate that is formed in dihydrobenzofuran isomerization, methyl-2-methylene-3,5-cyclohexadiene-1-one, is very stable despite the complete loss of resonance energy of the benzene ring. This was proven to be due to the formation of a very strong > C=O bond in the process. Rate constants based on the quantum chemical calculations using transition-state theory are in very good agreement with the experimental results.