Silica (Aerosil A200 V) has been functionalized by reaction with methacryloylpropyltrimethoxysilane (MPTMS) in toluene. The grafting yield first increases with the ratio MPTMS/SiOH and then levels off after full monolayer coverage, each molecule of MPTMS covering 71 Angstrom(2). A greater amount can be grafted if the MPTMS is first hydrolysed and partly condensed, and then reacted with the silica. A high degree of functionalization is needed to get complete dispersion of the silica in toluene. The redispersion of the functionalized silica in water is difficult; the best results are obtained in the presence of isobutanol as dispersing agent, after three days of treatment at pH 9.5, which causes the hydrolysis of the remaining methoxy groups.