Adsorption isotherms of N-2, water, NH3, pyridine, n-hexane and cyclohexane were determined and used to study the adsorption behavior on high silica, mesoporous molecular sieve MCM-41 and trimethylsilylated. MCM-41. X-ray diffraction (XRD), and Si-29 and C-13 magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) were used to characterize the surface composition and structure of the materials. These results and the comparison plot from the adsorption of nitrogen provide evidence that the wall surface of the mesopore is nonuniform. There are micropores with similar to 0.50 nm openings on the wall of the parent sample. Trimethylsilylation replaces similar to 79% of silanol groups in the parent sample, changes the surface composition and structure, destroys the acidic center, and blocks the micropore. Therefore it increases the hydrophobicity, contracts the pore size, greatly decreases the adsorption at low partial pressure, and changes the adsorption isotherms for various adsorbates. The process of capillary condensation in the mesoporous channels is discussed.