Aq. Na2SiO3 dosed into aq. 5% (A) NH4HCO3 or (B) (NH4)(2)CO3, 1:2 v/v, in the presence of 1 wt. pt. of nonylphenyl polyoxyethylene glycol ether (Rokafenol N-9) at similar to20degreesC, yielded SiO2 which next was surface-modified with 3-mercaptopropyl- or 3-methacryloxypropyl-trimethoxy silane, 0.5, 1, 2, 3 or 5 wt. parts (SiO2 basis). The type-B SiO2 had bulk d. 77-114 g/L; and modified with 3 or 5 wt. pts. of each silane had equally high bulk d. and reduced paraffin oil gain. The type-A SiO2 modified with 3 wt. pts. of the methacryl silane had bulk d. 36 g/L and showed paraffin oil and water gains of 1525 and 675 mL/100 g, resp. The methacryl silanes's longer chain enhanced hydrophobicity. At 5 wt. pts., it increased the av. particle size from 873 to 944 nm and polydispersity from 0.005 to 0.025, and eliminated secondary agglomerates.