THE synthesis of mesoporous siliceous solids with large-diameter channel apertures (25-100 Angstrom) has greatly expanded the capabilities of heterogeneous catalysis(1-7). The large apertures in such mesoporous silicas can, for example, be modified by framework substitution to create highly selective catalysts(4,5). Here we show that direct grafting of an organometallic complex onto the inner walls of mesoporous silica MCM-41 (ref. 1) generates a shape-selective catalyst with a large concentration of accessible, well spaced and structurally well defined active sites. Specifically, attachment of a titanocene-derived catalyst precursor to the pore walls of MCM-41 produces a catalyst for the epoxidation of cyclohexene and more bulky cyclic alkenes.