Hexadecyltrimethylammonium bromide-modified pptd. silica was calcined 7 hr./550 degrees C to yield mesoporous SiO2, av. pore diam. 2.8 nm, sp. area 1082 g/m(2) (originally 175). This SiO2 was used, 30 phr, to fill a butadiene-acrylonitrile rubber crosslinked with ZnO 5, stearic acid 1, S-8 2, and dibenzothiazole disulfide 3 phr. At pH 3-6 and 6-10, the s-potential of aq. dispersions of this mesoporous SiO2 varied from ca. -5 to -50 and leveled off at -50 mV, resp. Benzalconium bromide and triethanolamine as dispersants, 2 wt. parts/30 g filler, made v more pos. Particle size of mesoporous SiO2, was 58-65 nm. This SiO2 made cross-link d. greater (840) than did silica gel (413) and pptd. SiO2 (567 mu mol/cm(3)).