Abstract: A process for preparing hydrophobic finely divided particles of oxides of metals and/or oxides of silicon by chemically bonding hydrocarbon radicals to the surface of the oxide particles, said process comprising intensively mixing oxide particles having a water content less than about 1% by weight with a hydrolyzable metal or metalloid compound containing OR groups, wherein said compound is an ester of the general formula:Me(OR).sub.4wherein Me is an element from group IV of the periodic table, and each R is independently selected from C.sub.1 -C.sub.8 alkyl radicals or an aryl radical. A hydrophobic silicon dioxide particle is provided. Particles prepared by the process are useful as fillers in elastomers.