We report a novel sol-gel dip-coating process to form dual-layer microporous silica membranes with improved membrane performance and reproducibility. First, we deposit a surfactant-templated silica (STS) intermediate layer on top of a commercial gamma-alumina support both to improve its ＇surface finish＇ and to prevent a subsequently deposited microporous overlayer from penetrating into the support. Second, membranes are processed under clean room conditions to avoid dust contamination, and third, membranes are vacuum-calcined to promote further pore shrinkage and impart surface hydrophobicity. The resulting asymmetric membrane exhibits a gradual change in pore diameter from 50 Angstrom (gamma-alumina support layer) to 10-12 Angstrom (STS intermediate layer), and then to 3-4 Angstrom (30 nm thick, ultramicroporous silica top-layer). Compared to a single-layer process using only the microporous overlayer, the dual-layer process improves both flux and selectivity. For the industrially important problem of natural gas purification, the combined CO2 flux ((3-0.5)x 10(-4) cm(3)(STP)/s/cm(2)/cm Hg) and CO2/CH4 separation factors (200-600) are superior to all previously reported values for the separation of a 50/50 (v/v) CO2/CH4 gas mixture. In addition, the membrane selectively separated hydrogen from a simulated reformate from partial oxidation of methanol as evidenced by a high concentration of hydrogen recovery.