SiO2 (Aerosil) reacts with trialkoxymethanes and triaryloxymethanes (orthoformates) of the formula HC(OR)(3), R = CH3, C2H5, n-C4H9, and C6H5. The modified surface was analyzed with IR and solid-state NMR spectroscopy and thermogravimetric analysis (TGA). The results show that only the alkyl (or aryl) residues are left on the surface (as part of alkoxy or aryloxy groups). Alkyl residues are more tightly bound to the surface than aryl residues. The reaction of SiO2 with trialkoxymethanes allows stable modifications with organic products in the range of monolayer coverage, with little effort.