A headspace-GC (HSGC) method with multiple extractions from a single vial was developed for the measurement of desorption isotherms with any volatile organic component at any reasonable temperature. The geometric parameters of porous silica and chemically modified silica have been determined by this method. The methodology and instrumentation for the determination of porosity and surface areas of solids at various temperatures are described. This methodology is based on the periodic withdrawal and gas chromatographic analysis of aliquots of vapors from a headspace vial containing the adsorbent spiked with a chemical of interest (adsorbate). The instrumentation is calibrated using the same chemical in a vial without adsorbent. The desorption isotherm of the adsorbate on mesoporous silica was measured by headspace GC at 363 degrees K up to p/p(o) = 1.00 and by low-temperature nitrogen adsorption at 77 K. On the basis of the desorption isotherms, the pore size distribution was calculated by the Barrett, Joyner, and Hallender(1) method where the pore shape was assumed to be cylindrical. The measurement of the total surface area was performed according to Brunauer-Emmett-Teller (BET) theory.(2) The isotherm obtained using benzene as the adsorbate was then used for the determination of the geometrical properties of the mesoporous silica. The surface area, pore volume, and average pore radius were determined to be 325 m(2)/g, 1.1 mL/g, and 77 Angstrom for Prodigy-Si (Phenomenex. Torrance, CA) using benzene with the headspace-GC method. The surface area, pore volume, and average pore radius determined by low-temperature nitrogen adsorption were 326 m(2)/g, 1.2 mL/g, and 75 Angstrom, respectively. In addition to geometrical characterization of surfaces, the methodology and instrumentation proposed provide information about specific interactions between the adsorbate and the surface of the adsorbent or catalyst under study. It also permits the determination of the energies involved in these interactions as well as the evaluation of the competitive sorption of mixtures.