NOVELTY - In the production of a gas separation membrane (I) by depositing a film of silica sol on a porous support then thermally treating the film, the silica sol to be deposited in film form is prepared by hydrolyzing a silicon alkoxide in presence of a doping amount of a trivalent element oxide precursor (II).    USE - The use of (I)/(I') is claimed for the separation of helium or hydrogen from gaseous mixtures (specifically at temperatures above 250degreesC). (I) is especially useful for removing potentially corrosive impurities (such as carbon monoxide, carbon dioxide, methane or fission products (e.g. xenon and krypton)) from hot helium streams in the primary circuits of new generation high temperature nuclear reactors (HTR's). A nuclear plant having a helium heat carrier circuit, provided with a gas separation system for purifying helium which includes a membrane (I)/(I'), is independently claimed. Other specific applications include the extraction of hydrogen from gaseous effluents from petrochemical refineries; and purification of hydrogen (e.g. to remove catalyst posions such as carbon monoxide) before introduction into a synthesis reactor or fuel cell.    ADVANTAGE - The process gives a fault-free (specifically fissure-free), thermally stable microporous layer based on amorphous silica, providing a membrane ensuring efficient separation of gases (e.g. helium or hydrogen) from gaseous mixtures at 300-500degreesC. Highly selective gas separation can be obtained using very thin silica layers. The permeation and selectivity properties of the membranes are at least as good as (and generally superior to) those of membranes produced by prior art methods. Typically the permeation values of (I) for helium or hydrogen are ca. 10-6 mol.m-2.s-1.Pa-1. The advantageous properties of (I) are obtainable in a simple and reproducible manner; and are probably due to reduction of the degrees of crosslinking and rigidity of the silica due to introduction of the trivalent element into the silica lattice .    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a membrane (I) with a microporous layer of trivalent element-doped silica deposited on the support, obtained by the above process; and    (2) a gas separation membrane (I'), comprising a microporous boron-doped silica layer deposited on a mesoporous support (with no restrictions as to the production method).