The adsorption of argon, nitrogen, and carbon monoxide in porous Vycor glass has been studied by volumetric and microcalorimetric methods and by thermoporometry, Samples with particle sizes ranging from <50 to >200 mu m have been selected and treated by sample controlled thermal analysis (SCTA), Subsequent characterization indicates that the particle size has no influence on the pore texture and nature. Thermal treatment, however, modifies the chemical nature of the surface. It would seem that nitrogen and carbon monoxide assume a distinct mean orientation leading to smaller effective cross-sectional areas than those usually accepted. Carbon monoxide clearly distinguishes two different types of adsorption site for samples treated at low temperature.