Density functional theory study of propene metathesis proceeding on monomeric Mo-alkylidene centers of molybdena-alumina catalysts is reported. Calculations have been carried out with the Gaussian 98 program, using the hybrid B3LYP functional. The applied models of active Mo sites are bonded to alumina clusters including two or four aluminum atoms. According to the calculations, two kinds of the molybdacyclobutane intermediates play a role in the mechanism of propene metathesis: one with trigonal bipyramidal geometry and the second with a square pyramidal structure. The latter intermediate is formed from the former and this reaction competes with the decomposition of the trigonal bipyramidal molybdacyclobutane to the Mo-alkylidene center and alkene. Both syn and anti rotational isomers of Mo-ethylidene centers have been considered and their reactivities toward propene are almost the same. (C) 2003 Elsevier Inc. All rights reserved.