A silica aerogel prepared at 37degreesC from Si(OEt)(4) as precursor with 0.74N NH4OH and 0.1N NH4F as catalysts in abs. EtOH as solvent, Si(OEt)(4):H2O:NH4OH:NH4F = 100:400:0.8:0.15 by motes, density 70 kg/m(3) (recalcd. per SiO2), was deposited on Schleicher-Schuell borosilicate fiber GF55 substrate, maturated 2 days in abs. EtOH, and dried with supercritical CO2. Three filters thus prepared were hydrophobized with Me3SiCl vapors for (A) 1, 2, and 3 days and matured at room temp. for 1, 3, and 7 days, resp. Contact with water showed filter A to have been hydrophoblzed. The longer was the Me3SiCl treatment, the more destroyed was the hydrophobized surface, cracked and thinner than before the treatment.