Grafting of [W( NAr)(=CHtBu)(2,5-Me2NC4H2)(2)] on a silica partially clehydroxylated at 700 degrees C (SiO2-(700)) generates the corresponding monosiloxy complex [( SiO)W( NAr)(=CHtBu)(2,5-Me2NC4H2)] as the major species (approximate to 90%) along with [( SiO)W( NAr)(CH(2)tBu)(2,5-Me2NC4H2)(2)], according to mass balance analysis, IR, and NMR studies. This heterogeneous catalyst displays good activity and stability in the metathesis of propene. Very importantly, solid state NMR spectroscopy allows observation of the propagating alkylidene as well as stable metallacyclobutane intermediates. These species have the same reactivity as the initial surface complex [( SiO)W( NAr)(=CHtBu)(2,5-Me2NC4H2)], which shows that they are the key intermediates of alkene metathesis.