Grafting (ArO)(2)W(=O)(=CHtBu) (ArO = 2,6-mesitylphenoxide) on partially dehydroxylated silica forms mostly [( SiO)W(=O)(=CHtBu)(OAr)] along with minor amounts of [( SiO)W(=O)(CH(2)tBu)-(OAr)(2)] (20%), both fully characterized by elemental analysis and IR and NMR spectroscopies. The well-defined oxo alkylidene surface complex [( SiO)W(=O)(=CHtBu)OAr] is among the most active heterogeneous metathesis catalysts reported to date in the self-metathesis of cis-4-nonene and ethyl oleate, in sharp contrast to the classical heterogeneous catalysts based on WO3/SiO2.