Process for the silylation of inorganic oxides

Highly apolar, pyrogenic silica having an average primary particle size of less than 100 nm, with a specific surface area of greater than 25 m.sup.2 /g, a carbon content of at least 1% by weight per 100 m.sup.2 /g of specific surface area, a methanol number of greater than or equal to 50, a relative sorption capacity for hydroxyl ions of less than 15%, and no fraction of silylating agent which is soluble or extractable from the silica, and in that no isolated silanol groups can be detected at a wavenumber of 3750 cm.sup.-1 on the highly apolar pyrogenic silica by means of infrared spectroscopy, wherein said highly apolar, pyrogenic silica is prepared by treating a very finely divided silica with at least one silylating agent which is relatively nonvolatile in the temperature range of the overall process, with a proviso that the relatively nonvolatile silylating agent is admixed as a liquid with the finely divided silica, in the form of a very finely atomized aerosol.