Density functional theory calculations were carried out to evaluate the potential energy surfaces of the unimolecular isomerizations of 2,3-dihydrofuran and 5-methyl-2,3-dihydrofuran and the interisomerization between the isomerization products. Equilibrium and transition state structures were optimized by the Lee-Yang-Parr cot-relation functional approximation (B3LYP) using the Dunning correlation consistent polarized double xi basis set. Energy values at critical points were calculated at the QCISD(T) level of the theory. Isomerization rate constants were calculated using transition state theory and were compared with the experimental results. The agreement between the calculated and the experimental rate constants are in most cases very good.