NOVELTY - A supported zeolite membrane (I) comprising a porous composite layer of zeolite/support, shows an ideal permselectivity for N2/SF6 of more than 45, preferably more than 80.    USE - For the separation of gases, vapors or liquids, especially (a) linear and branched 4-8C hydrocarbons (saturated or unsaturated) (more especially linear and branched isomers, linear and branched paraffins or linear and branched olefins), (b) aromatics/naphthenes, (c) paraffins and olefins, (d) linear hydrocarbons and naphthenes, especially paraffins and naphthenes, (e) linear hydrocarbons (especially paraffins) and aromatic hydrocarbons, (f) branched hydrocarbons according to their degree of branching (especially the separation of mono- and di-branched and/or multi-branched compounds), (g) isomers of xylene, (h) gas mixtures such as methane/nitrogen, methane/carbon dioxide, methane/carbon monoxide, CO2/CO or nitrogen/oxygen, (i) methane and sulfur compounds, especially methane/hydrogen sulphide or methane/COS, (j) hydrogen/hydrocarbons, nitrogen/hydrocarbons, hydrogen/CO2 or hydrogen/CO and (k) alcohols and water (claimed).    ADVANTAGE - A supported zeolite membrane with a very high ideal nitrogen/sulfur hexafluoride permselectivity or selectivity and a very high nitrogen permeability    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method for the production of (I) by (a) making a gel or solution containing water and a source of silicon plus organic structuring compound(s) and possibly another framework element such as aluminum, boron, iron, titanium, phosphorus or gallium and a mineralising agent such as a source of hydroxide, (b) bringing the gel or solution into contact with a porous support, (c) crystallising the zeolite from the gel or solution and (d) eliminating residues by calcination; in this process, stage (c) is carried out with stirring, with a water/silicon mol ratio of 10-100, preferably 60-80, and the porous support is pretreated in such a way as to render certain porous zones or external surfaces unavailable for the formation of zeolite crystals