Density functional theory (DFT) study of the pathway of ethene metathesis proceeding on monomeric Mo-VI centres of MoO3/Al2O3 catalyst is reported. Calculations were carried out with the GAUSSIAN 98 program using the hybrid B3LYP functional. Models of active Mo-VI sites including alumina cluster of formula Al-2(OH)(6) were used. Ethene addition to molybdenamethylidene centre leading to the trigonal bipyramidal molybdacyclobutane as well as the conversion of the trigonal bipyramidal molybdacyclobutane to the square pyramidal structure were theoretically investigated. It was concluded that the last reaction competes with the decomposition of the trigonal bipyramidal molybdacyclobutane to molybdenamethylidene centre and ethene. (C) 2002 Elsevier Science B.V. All rights reserved.