High silica MCM-41 and modified samples were characterized in detail with XRD, ^2^9Si and ^1^3C CP MAS NMR, and adsorption isotherms of nitrogen, water and cyclo-hexane. The modification reagents used were trimethylchlorosilane, dimethyldichlorosilane, methyltrichlorosilane, silicon tetrachloride and methyliodide. The modification greatly varies the surface structure, decreases the pore diameter, and increases the wall thickness. A linear relationship between the contents of silanol groups and the amounts of water and cyclohexane absorbed reveals the nature of adsorption centers in the samples.

Key words: NMR SPECTROMETRY, MODIFICATION, SURFACE STRUCTURE, ADSORPTION, PORE SIZE DISTRIBUTION, CHLOROSILANE, IODOMETHANE