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MCM-48 was surface modified via vapor-phase reactions with hexamethyldisilazane (CH 3 -MCM-48) and 3-aminopropyldimethylethoxysilane (NH 2 -MCM-48). 29Si NMR confirmed that the resulting materials contained covalently attached trimethylsilane and 3-aminopropyldimethylsilane moieties, both important functionalities for bioseparation applications. The surface coverage was ∼1.8 and 0.9 groups per nm 2 , respectively. The X-ray diffraction patterns and the narrow pore size distributions obtained from the gas sorption isotherms showed that the modified materials retained the characteristic pore structure of the underlying MCM-48 material. CH 3 -MCM-48 exhibited significantly improved hydrolytic stability over the unmodified MCM-48 under the aqueous conditions tested, whereas NH 2 -MCM-48 appeared to be less stable than the unmodified MCM-48. The decrease in stability is most likely due to the nature of the attachment of the 3-aminopropyldimethylsilane moiety, where the conversion of surface silanol groups is limited by H bonding with the amino end, leading to a 50% lower surface concentration and resulting in an increased likelihood of nucleophilic attack on the silica surface, enhancing the rate of hydrolysis. Hexamethyldisilazane thus appears to be a superior functional group for modifying the MCM-48 surface.