Abstract: Highly dispersed metal or metalloid oxides are rendered hydrophobic byI. contacting such oxide particles while in the form of an aerogel with a dry inert gas stream in a fluidizing bed at a temperature from 600.degree. to 1000.degree. C and at atmospheric pressure for a period of less than 60 seconds to render the particles absolutely dry and thus activate them;Ii. charging the resultant dried and activated particles in a fluidizing bed with at least one gas-phase organosilicon compound selected from the group consisting of (a) linear organopolysiloxanes (b) cyclic organopolysiloxanes (c) a mixture of both types of siloxanes and (d) a mixture of any of the substances (a)-(c) with an organohalogenosilane at a temperature in the range from 25.degree. to 650.degree. C;Iii. causing said charged particles to react with the organosilicon compounds at a temperature from 35.degree. to 650.degree. C; andIv. finally treating the reaction product with an inert gas stream in a fluidizing bed at temperatures from 500.