Degenerate propene metathesis proceeding on Mo-alkylidene centres situated on alumina is theoretically investigated, applying the B3LYP functional. The Gaussian 98 package is used. The present results are compared with previous ones, concerning the productive metathesis. It is concluded that on the Mo-ethylidene sites the degenerate metathesis is preferred over the productive one. This is not the case for the Mo-methylidene centres, however. According to the calculations, the population of the Mo-ethylidene centres should be higher than in the case of the Mo-methylidene ones and the formers are the predominant chain carriers for the degenerate metathesis. The relative energies of mono-methyl substituted. as well as, 1,2- and 1,3-dimethyl substituted trigonal bipyramidal and square pyramidal molybdacyclobutanes are also compared. (C) 2004 Elsevier B.V. All rights reserved.