Chemical vapor deposition (CVD) was used to modify 4 nm pore, sol-gel derived, gamma -alumina membranes supported on macroporous alpha -alumina. Aluminum oxide was deposited in the pores of the gamma -alumina membrane by alternating additions of trimethylaluminum (TMA) and water vapor. By reducing the pore size, the permeance of non-condensable gasses was reduced much more than the permeance of condensable gasses due to capillary condensation or preference adsorption of water vapor. The modified membrane that exhibited the best separation properties had a water vapor permeance ranging from 1.5 x 10(-6) to 3.0 x 10(-7) mol/m(2) s Pa, an oxygen permeance ranging from 1.7 x 10(-7) to 1.5 x 10(-9) mol/m(2) s Pa, and a separation factor as high as 140 at room temperature. The microstructure of the pores contained some irregularities which were attributed to an atomic layer CVD (ALCVD) mechanism modified by homogeneous reactions. The effect of the modified ALCVD was higher permeances than would be expected. P-type zeolite membranes were also made and found to have similar separation properties to the more heavily modified gamma -alumina membranes.