Mesoporous silica membranes, prepared by surfactant-templating with a pore diameter of ∼4 nm on an alumina support, are modified by atomic layer deposition (ALD) of aluminum oxide. ALD of aluminum oxide is achieved using trimethyl aluminum (TMA) and water as reactants. Membranes modified up to 50 ALD reaction cycles are characterized periodically during progressive reaction cycles to provide details of the pore modification process. A decrease in light gas permeance, pore size distribution shift to lower pore sizes, and a decrease in porosity provide evidence for pore-size reduction through ALD. Further analysis of the data indicates that the ALD reaction favors larger pores and defects, and becomes less efficient as the pore size decreases.