Alumina oxide was deposited in the straight nanosize pores of Anopore alumina membrane by the alternative introduction of trimethylaluminum (TMA) and water vapor to the membrane pores. The chemical vapor deposition (CVD) modification was conducted under two different schemes: (1) cyclic CVD with residual pressure and (2) cyclic CVD with purge. The membranes were characterized by single gas helium permeance and water vapor/oxygen multiple gas permeation. Helium gas permeation results show that the modification by the residual pressure scheme caused greater reduction in the pore size than porosity, suggesting the deposition of alumina in a fractal structure in the Anopore membrane pores. Such a structure is caused by combined homogeneous and heterogeneous reactions taking place respectively on the pore wall and randomly in the pores. The membranes modified with CVD under the purge scheme exhibited greater reduction in porosity than the pore size, indicating that the alumina in the latter case was deposited on the pore wall in an atomic layer fashion. The structure in the latter case is caused by homogeneous reactions taking place on the pore wall. Membranes modified under the purge scheme exhibited improved water vapor/oxygen separation properties.