Hydrophobized silicon-iron mixed oxide powder, characterized in that it has the following physicochemical characteristics: BET surface area 20 to 75 mVg; Carbon content 0.5 to 10% by weight; Tamped density 150 to 600 g/l; Chlorine content 0.1 to 3.0%; Drying loss 0.1 to 4% by weight; DVS isotherm (60%) 0.5 to 1.5% by weight; Heating rate (Is, 10%) 50 to 550° C./s; 90% range (number) 5 to 50 nm; 90% range (weight) 5 to 150 nm; Overall range 2 to 200 nm, is prepared by treating a silicon-iron mixed oxide powder with the surface modifier either in spray form or in vapour form, and then heat treating it. The surface-modified oxidic particles can be used as a filler in adhesives. Further fields of use are use for data carriers, as a contrast agent in imaging processes, for biochemical separation and analysis processes, for medical applications, as an abrasive, as a catalyst or as a catalyst support, as a thickener, for thermal insulation, as a dispersing aid, as a flow aid and in ferrofluids.