This work reports on the hydrothermal synthesis and gas-vapor separation properties of ordered mesoporous MCM-48 and Al-MCM-48 membranes. Gas permeation data for these membranes is characterized by the Knudsen type permeation mechanism, with a helium permeance of 7 x 10(-8) mol/m(2) Pa s and 3 x 10(-8) mol/m(2) Pa s, respectively, confirming the mesoporous nature of the membranes synthesized. Separation of a water vapor/oxygen mixture for these membranes was studied to explore potential use of these membranes in air dehumidification and air conditioning applications. These membranes offer high water vapor/oxygen selectivity as a result of capillary condensation of water vapor in the pores of the membranes. In the case of MCM-48, an oxygen permeance of 5 x 10(-9) mol/m(2) Pa s is obtained, with a water permeance of around 1 x 10(-7) mol/m(2) Pa s. The water vapor/oxygen separation factors as high as 17 are achieved. Al-MCM-48 membranes are seen to provide much higher separation factors reaching as high as 142, with water vapor permeance of 6 x 10(-8) mol/m(2) Pa s. (c) 2010 Elsevier B.V. All rights reserved.