Mesoporous γ-alumina membranes were chemically modified with several organochlorosilanes. XPS with depth profiling and permporometry experiments were used to study the local organosilane concentrations and their effect on the pore structure. It was found that the penetration of long chain chlorosilanes into the mesopores was sterically hindered and that grafting was not as effective as for short chain organosilanes. Membranes modified with monofunctional organosilanes showed a smaller Kelvin radius than unmodified γ-alumina membranes as observed by permporometry, indicating that methyl groups resided inside the pores. For membranes modified with di- and trifunctional organosilanes larger Kelvin radii were observed, indicating that polymerized silane networks block the smaller pores, thereby decreasing the overall permeability of the membrane. The permeability of hydrophilic and hydrophobic solvents was affected by the hydrophobic modification of the surface and the internal pore walls depending on the nature of the chlorosilane used for grafting.