The structural stability of mesoporous molecular sieves organically functionalized by a direct method has been investigated in terms of the stability against moisture treatment and mechanical pressure. Incorporation of organic groups directly attached to Si atoms by the use of organosilanes results in an enhancement of the structural stability compared to purely inorganic ones, this enhancement could be ascribed to an increase in hydrophobicity. By pH adjustment and hydrothermal treatment, a further improvement in the stability is attained. A clear relationship between the structural stability and the amount of monolayer adsorption capacity for water is observed. Organically functionalized samples show a lower monolayer adsorption capacity for water than purely inorganic materials, even though both have the same amount of silanol groups, this is probably due to the water-repelling ability of the surface organic groups.