The active sites (15% of total Re) of CH3ReO3 Supported on alumina arise solely from the reaction of the C-H bond of CH3ReO3 with reactive Al-S-O-S sites of a gamma-Al2O3 surface to yield [AlSCH2ReO3], whereas the major species, inactive, corresponds to MeReO3 chemisorbed through its oxo ligand(s) on Lewis acid sites of alumina. Monitoring the active sites of CH3ReO3 supported on alumina by solid-state NMR spectroscopy shows that no carbenic signals are observed and that the resting states of the catalyst in the metathesis of propene correspond to mu-methylene and mu-ethylidene species, so that the propagating carbenic species are likely formed only in the presence of olefins. Although this system is highly active in the metathesis of propene, with initial rates similar to some of the best catalysts, it undergoes fast deactivation, which is due to the propene metathesis products, most likely ethene. (C) 2007 Elsevier Inc. All rights reserved.