Hydrophilic organic xerogels were transformed into hydrophobic materials by treating them with hexamethyldisilazane. This transformation was performed by a simple one-pot process without modifying the previously designed porosity of the xerogel, at very soft operating conditions (80 degrees C and atmospheric pressure) in the absence of catalyst or any other additional compound. Hexamethyldisilazane reacts with dangling hydroxyl surface groups, blocks the oxygen with methylsilane groups, and prevents the formation of hydrogen bonds with water molecules. The reaction proposed was corroborated chemically by X-ray photoelectron spectroscopy and thennogravimetric analysis coupled with a mass spectrometer. The porous structure of the organic xerogel determines the minimum time for the surface modification due to steric hindrance. Therefore, the former hydrophilic surface of the organic xerogel may be transformed into a hydrophobic one by a simple process, thus leading to the possibility of designing not only the porous structure but also the surface chemistry of the resorcinol formaldehyde xerogels to fit the requirements of an application.