The fabrication process is described of supported microporous Si3N4 membranes, prepared by pyrolytically decomposing organo-substituted polysilazane precursor. The membrane had a composite asymmetric structure consisting of a mechanically strong porous Si3N4 support which had 42 vol% pores between 0.4 and 0.52 mu m, coated with an intermediate and one or two thin active top layers. The individual layers were fabricated by the conventional dip-coating technique. Permeation experiments with He, N-2 and CO2 have been performed to determine the gas transport characteristics and separation performance of the processed membranes. The permeation is pressure-independent, indicating no viscous flow in the supported top layer. The proposed process has made it possible to prepare membranes with He permeation rates of greater than or equal to 5.3 x 10(-6) mol m(-2) s(-1) Pa-1 and He/N-2 permselectivities of greater than or equal to 2.0, even in the membrane with one top layer. It is also demonstrated from separation experiments, that the membrane with high quality top layer has the separation factors of 4.7 for He/N-2 and of the theoretical of Knudsen flow for CO2/N-2