A surface conversion technique was developed to make the SiO2surface extremely hydrophobic. Two kinds of fluorine-substituted organosilanes, (heptafluoroisopropoxy)propyl methyl dichlorosilane and trifluoropropylmethyl dichlorosilane, were utilized as the conversion agents. The hydrophobicity was evaluated in terms of the London dispersion force component of the surface free energy (Fowkes's γds) by measuring the contact angles of CH2I2, hexadecane, and H2O. The values of γdsfor the converted surface are 15 and 21 mJ/m2for the former and the latter conversion reagent, respectively. These surfaces were found to have resistance to 60 min hydrolysis in boiling water and pyrolysis in air up to 450°C.