The piezogravimetric method has been used to study the adsorption of dimethyl and trimethylchlorosilanes on quartz crystal surfaces by varying the modes of treatment. The nature of the resulting low-energy surfaces has been discussed in the light of the extent of adsorption of n-hexane and water, calculated at different relative pressures, from their isotherms. The results obtained permit the conclusion that after the adsorption of silanes at 240 °C, the modified surface may be described as close-packed methyl surface, impenetrable to n-alkane and water vapour.