Pathways of ethene metathesis proceeding on two Mo-methylidene sites differently located on the (1 0 0) surface of gamma-Al2O3 are investigated with DFT calculations, applying a relatively large cluster model of alumina. Convergence of the electronic properties with respect to the cluster size is achieved for clusters as large as Al8O26H28. It is shown that location of the active site influences its reactivity towards ethene. When the Mo-methylidene centre replaces two basic hydroxyl groups, ethene addition, leading to a trigonal bipyramidal molybdacyclobutane, is an endothermic process with relatively high activation energy. On the other hand, when only one basic OH group is replaced by the Mo centre and the molybdenum atom is also directly bonded to a bridge oxygen of the alumina surface, electron density on the Mo-methylidene moiety is reduced, comparing to the first case, and also the geometry of the site is more suitable for alkene addition. Ethene attack on this molybdenum site results in formation of a stable pi-complex that can further rearrange with a low energy barrier to the trigonal bipyramidal molybdacyclobutane. For both pathways investigated, it is predicted that rearrangement of the trigonal bipyramidal intermediate to a much more stable square pyramidal molybdacyclobutane proceeds easier than continuation of the catalytic cycle of ethene metathesis. (c) 2005 Elsevier B.V. All rights reserved.