Disilazane reagents of type HN(SiR1R22)2 carrying organic substituents R1,2 = H, Me, Ph, n-Bu, n-Oct, Vin (Vin = vinyl) of varying longitudinal and lateral extension are reacted with high quality MCM-41 samples of different pore sizes, i.e., effective pore diameters of 2.8 and 3.8 nm according to the BJH pore size analysis of the desorption branch. The reaction of the standard silylating reagent hexamethyldisilazane, HN(SiMe3)2, is shown to be controlled by the amount of added silylamine and the contact time, resulting in effective pore size engineering. Calculations from elemental analysis revealed that the degree of silylation (silylation efficiency) and hence the surface hydroxyl consumption depend on the steric bulk/shape of the groups R. The surface coverage varies from 0.74 to 1.85 silyl groups/nm2. The sterically least demanding tetramethyldisilazane, HN(SiHMe2)2, is the most efficient silylating reagent, while silyl groups with bulky phenyl substituents produce the lowest surface coverage featurin...