Quantum chemical calculations on the mechanism of the beta-scission reaction in zeolites were performed using density functional theory and Hartree-Fock methods. The results obtained indicate that the potential energy surface for this reaction is very complex. Three reaction paths were identified : path RL, one-step via the "ringlike" transition state (TS); path HBCP, via the "hydrogen-bonded" TS and substituted cyclopropane : and path HB, one-step via the "hydrogen-bonded" TS, Transition states in all reaction paths represent complexes of the carbocation-like fragment with the negatively charged cluster, whereas both initial and final states represent alkoxy species with a covalent bond between a carbon atom of the hydrocarbon portion and a zeolite oxygen. The dependence of calculated activation energy on the cluster model of zeolite and on the calculation level is discussed. The B3LYP/6-31++G**//B3LYP/6-31G* activation energies for p-scission of but-1-oxy and pent-2-oxy with the H3Si(OH)AlH2(OSiH3) cluster were found to be 57.3 and 52.3 kcal/mol, respectively.