The mechanism of propylene oxide isomerization, to yield four isomerization products, was calculated by the B3LYP/cc-pVDZ method. Coupled cluster CCSD(T) calculations for the reactant, products, transition states, and intermediates were carried out to estimate the activation energies. The vibrational frequencies, calculated using B3LYP method, were used to estimate transition state theory frequency factors. The potential energy profiles of the isomerization of propylene oxide to acetone, propanal, and allyl alcohol contain one transition state each, indicating that these reactions proceed via concerted mechanisms with simultaneous C-O bond rupture and 1,2- or 1,4-H-atom shift. The potential energy surface of the isomerization of propylene oxide to methyl vinyl ether contains three transition states and two intermediates. The initial step of the process is a C-C bond cleavage with a relatively low barrier. The first intermediate has a trans structure, and the second intermediate on the surface has a cis structure. The cis --> trans isomerization is a key issue in the isomerization process. The last stage is a 1,4-H-atom shift. The rate constants calculated at the coupled cluster CCSD(T)/cc-pVDZ//B3LYP/cc-pVDZ level of the theory show a fair agreement with the experimental values.