The invention relates to surface-modified nanoparticles composed of metal or semimetal oxides or hydroxides and having been reacted on their surface with a modifier, with formation of covalent bonds. The modifier is a polysiloxane. The invention also relates to a process for preparing the nanoparticles, to their use as fillers in coating materials, plastics, foams and nail varnishes, and to dispersions which comprise the nanoparticles.