Supported molybdenum oxide on e.g. alumina is a useful catalyst for metathesis of olefins. However, the presence of acid sites will influence the selectivity of the metathesis reaction, since they may catalyze double bond isomerisation of olefins. The acidity of supported molybdenum oxide on alumina was modelled using neutral clusters. DFT calculations showed that the strength of acidity of the sites decreases and the activation energy for double bond isomerisation increases with a decrease in the formal oxidation state of the molybdenum oxide. (c) 2006 Elsevier B.V. All rights reserved.