Pyrogenic SiO2, 7 nm in original particle size, sp. surface 380 m(2)/g, was used to fill ethylene-propylene copolymer (EPM) (41% of propylene mers) and butadiene-actylonitrile rubber (-CN groups, 21%.) crosslinked with dicumyl peroxide and with S in presence of mercaptobenzthiazole, resp., and dispersed with 10% (SiO2 wt. basis) of cetyltrimethylammonium Br (I), benzalkonium Br (II), TEA or PEG nonylphenyl ether (III) alone or in blends thereof, and vulcanized at 160 degrees C. In a 0.2 g SiO2/g dispersion, homogenized ultrasonically for 1 hr., SiO2 agglomerates were 70150, 35; 160-470, 41; 1650-3000 nm, 21 vol.%. Surfactant, 10%, reduced the agglomerates to: I, 28-41, II 13-18, TEA 11-14, III or 13-16 nm. The effect was greatest in paraffin oil. The surfactants protracted vulcanization time, affected the xi-potential with rising pH (I gave xi > 0 over pH 3-13); III increased much the elongation-at-break but tensile strength only slightly.