The surface energetic heterogeneity of Aerosil OX50 silica is investigated before and after modification by the grafting of various amounts of hydrophobic hexadecyltrichlorosilane (HTS). A detailed study is conducted to obtain new insight into the grafting process using volumetric techniques based on molecular probing. The surface of the silica particles has been thoroughly characterized using classical N-2 Brunauer-Emmett-Teller measurements and by low-pressure quasi-equilibrium volumetry using nitrogen and argon as probe molecules. The experimental data are analyzed using the derivative isotherm summation method in order to gain quantitative information on the specific surface area, the surface energy and the polarity of the bare and HTS-covered silica, in terms of adsorption energy sites and heterogeneity. The modification process is also studied by Fourier transform infrared spectroscopy (FTIR) and total carbon mass measurements performed on solid samples (TOC). The combination of these three techniques leads to a better description of organosilane grafting onto the particles. The bare silica surface appears energetically heterogeneous due to the use of 5 different adsorption sites to describe the low pressure adsorption results. The HTS molecules are mainly located on the polar high and medium energy domains. At the largest HTS coverage (2.1 A mu mol/m(2)), the surface becomes energetically homogeneous (one site of low energy) and fully apolar. The data obtained from the low-pressure gas adsorption have been employed to estimate the grafted organic content in contact with the gas. The surface coverages derived from volumetry analysis and TOC measurements are in acceptable agreement but some difference can be recognized. The discrepancy is discussed in terms of conformation of the HTS molecules at the solid surface. The proposed conformational changes are supported by the FTIR results.