A periodic DFT approach was used to investigate ethene metathesis on a molybdena-alumina catalyst. Ten potential monomeric Mo-methylidene centers variously located on the (100) and (110) surfaces of gamma-alumina were modeled. The Mo sites differed from one another in terms of geometry, the number and type of Mo-O-Al linkages, and the number of free Mo = O bonds. The thermodynamic stability of the Mo-methylidene species was compared over a wide range of temperatures, taking into account the hydration state of the surface. Depending on water vapor pressure, structures bonded differently to the surface are stable. Moreover, the Mo sites on (110) gamma-alumina are more stable than their analogues located on the (100) face. The pathways of ethene metathesis proceeding on the Mo-methylidene centers also were studied. The various Mo species demonstrate varying activity. Most of the Mo-methylidene centers are reactive with ethene and form molybdacyclobutane complexes, which can exist as two coordination isomers with possible interconversion. In many cases, the molybdacyclobutane species are significantly more stable than the reactants. Thus, metallacycle opening to restore carbene and olefin is the rate-limiting step for ethene metathesis. The large majority of the Mo sites, although reactive with ethene, are blocked at the molybdacyclobutane step, and their catalytic reactivity is small at room temperature. Only a small minority of sites are catalytically active. It is concluded that the most active site is located on the (100) surface and is stable in dehydrated conditions. At higher temperatures, however, other Mo species can be active in olefin metathesis. (C) 2008 Elsevier Inc. All rights reserved.