Interior hydroxyl groups of molecular sieves MCM-41 and MCM-48 are investigated by NMR, FTIR and TG/DTA. The results show that in both mesoporous materials, 26-30% of the Si atoms carry OH groups. The internal surface is hydrophilic. Exchange experiments with D-2 and NH3 adsorption studies show that these silanol groups are freely accessible and not acidic. MCM-48 contains more associated terminal OH groups than MCM-41 which could result in different reactivities towards grafting agents. Beside the silanol groups, Bronsted-acid sites are aluminum substitution.