Conversions of acetaldehyde to hydroxyethylene, acetone to 2-hydroxypropylene, butanone to 2-hydroxybutene and 2-pentanone to 2-hydroxypentene catalyzed by H-ZSM-5 have been theoretically studied using quantum chemical methods. Geometry optimizations of the local structures of species reacting with H-ZSM-5 zeolite using 3T-DFT and 5T-DFT cluster models computed at B3LYP/6-31G(d) level and 50/3T-ONIOM2 cluster model computed at ONIOM(B3LYP/6-31G(d):AM I) level have been carried out. Three steps of the reaction mechanism were found and thermodynamic properties of each reaction steps and equilibrium constants of overall reaction have been obtained. The overall reaction of the conversion for all systems is endothermic reaction. The activation energies of all conversion reactions derived at three different methods are reported. (c) 2005 Elsevier B.V. All rights reserved.