Inorganic membranes consist of a stand-alone or supported material with special transport properties, able to operate at elevated temperatures and pressures. Inorganic membranes are classified according to the presence and size of connected porosity. Dense membranes have 100% selectivity for H-2 and O-2, and acceptable flux at high temperature. Micro-porous membranes have < 2 nm circle divide pores and combine high but < 100% H-2 selectivity with higher fluxes at lower temperatures. Meso-porous membranes have 2-50nnn circle divide pores, a good thermo-chemical stability, very high fluxes over a wide temperature range, and moderate Knudsen selectivity for separation of light molecules (H-2) from heavier ones (CO2). Meso-porous membranes are also considered for pressure-driven water purification. Large scale introduction of inorganic membranes has been hindered by lack of structural definition, reproducibility, and accurate design parameters. The present paper shows how these problems can be addressed by synthesis and characterization of smooth, strong and surface-defect free supporting structures. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.