The formation is described of supported microporous membranes (by IUPAC definition r(pore) < 1 nm), prepared by modification of mesoporous gamma-alumina membranes with polymeric sols. The mesoporous gamma-alumina membranes, with a top-layer thickness in the order of 7-10 mu m, and with pore radii of 2-2.5 nm, have a very high surface finish (mean roughness 40 nm). The amorphous microporous top-layer thickness is in the order of 60-100 nm. Gas transport properties are effectively improved as is shown by activated permeation and molecular sieve-like separation factors in the order of 50-200 for H-2/CH4. These microporous top-layers can be prepared from a relatively wide range of sol structures; from inorganic oligomers which are too small to result in significant scattering with SAXS, to polymeric structures with fractal dimensions in the range : 1 < d(f) < 2.04, and radii of gyration between 0.8 and 4 nm.