Quantum-chemical modelling of the heterogeneous oxidation of hydrocarbons on an oxide catalyst can be divided in three steps : a description of the interaction of the hydrocarbon molecule with the surface of the oxide cluster, an analysis of the bonding of oxygen molecules, and a description of the reaction of oxygen and hydrocarbon molecules. Quantum-chemical calculations of the reaction of benzene and toluene at the surface of a vanadium oxide-based catalyst modelled by a V6O20 cluster permit general conclusions to be formulated that pathways followed by catalytic reactions depend on the steric arrangement of the reacting molecules, which may be imposed by the properties of the active centre al the catalyst surface, its location on different crystal faces and by the type of frontier orbitals and the electronic state of the reacting molecules. Catalytic transformation is a concerted reaction in which interactions developed on the approach of reacting molecules to active centers cause the redistribution of electrons followed by the rearrangement of nuclei of both reactants and catalyst (reconstruction of the catalyst surface) and result in the desorption of the product.