TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Materials: The zeolite phase is of the MFI structural type and the porous support is selected from ceramic, alumina, zirconia, silica, titanium oxide, carbon, metal, polymer or a mixture of these. Preferred Product: (I) shows (a) an ideal N2/SF6 permselectivity of at least 48, preferably at least 81, and a permeability to nitrogen of at least 22 x 10-7 (preferably at least 30 x 10-7) mol/s/m2/Pa, or (b) an ideal N2/SF6 permselectivity of at least 60, preferably at least 93, and an N2 permeability of at least 34 x 10-7 (preferably at least 43 x 10-7) mol/s/m2/Pa. In the separation of n- and iso-butane, (I) shows (a) a permeability to n-butane of at least 3.75 x 10-7 (preferably at least 4.4 x 10-7) mol/m2/s/Pa and a permeability to isobutane of at most 0.4 x 10-7 (preferably at most 0.35 x 10-7) mol/m2/s/Pa, or (b) a permeability to n-butane of at least 5.50 x 10-7 (preferably at least 5.80 x 10-7) mol/m2/s/Pa and a permeability to isobutane of at most 0.50 x 10-7 (preferably at most 0.45 x 10-7) mol/m2/s/Pa (all measured at 175degreesC)    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Materials: Suitable organic structuring agents comprise quat. ammonium compounds (e.g. tetrapropyl-, triethylpropyl-, tripropylmethyl-, tributylmethyl-, tributylethyl-, tributylhexyl-, tributyloctyl-, trimethylbenzyl- or diethylpropylethyl-ammonium, or N,N,N,N',N',N'-hexamethyl-1,6-hexane-diammonium compounds), amines (e.g. n- or iso-propylamine, 3-dimethylamino-2,2'-dimethyl-1-propanol, 1,5-diaminopentane, 1,6-diaminohexane, 1,12-diamino-dodecane, ethanolamine, diethanolamine, dimethanolamine, monopropanolamine (n- or iso-), morpholine, N-ethylpiperidine), alcohols (e.g. 1,6-hexanediol, pinacol, 1,2-dimethyl-1,3-propanediol, glycerol, 1,2-propanediol, triethylene glycol, 1,4-cyclohexanedimethanol), ethers (e.g. mono-, di-, tri- or tetra-ethylene glycol dimethyl ether, THF, diethyl ether) and other compounds such as iodide, 1-methyl-4-aza-1-azoniabicyclo(2.2.2)octane 4-oxide, oxyethyllactamide, EDTA, nitrilotriacetic acid, tripropylamine N-oxide, hydroquinone, carboxymethyl-cellulose, hydroxyethyl-cellulose, sodium n-dodecylbenzenesulfonate and sodium n-alkyl-polyoxyethylene sulfate Preferred Method: Pretreatment of the support involves covering it with a solid such as a polymer film, a liquid such as water and/or a gas under pressure. Crystallisation (c) is carried out at 50-250 (preferably 150-200) degreesC, with a water/silicon mol ratio of 66-100, preferably 70-80; calcination (stage d) is carried out at 400-800 (preferably 450-600) degreesC. In stage (a), the mol ratio between the polar organic compound and the silica in the gel or solution is 0.2-0.8, preferably 0.3-0.5. EXAMPLE - A mixture of 27.3 g of a 20% solution of tetrapropylammonium hydroxide (TPAOH) and 68.7 g water was treated slowly with 4.03 g Aerosil 380 (RTM: pyrogenic silica), stirred to give a clear solution and left to stand for 3 days at room temperature (RT) to give a mixture with the molar composition 1 SiO2 - 0.4 TPAOH - 75 H2O. This mixture was then placed in an autoclave so as to cover a porous support in the form of hollow alpha-alumina tubes (Pall-Exekia T1-70 (RTM; 15 cm x 0.7 cm i.d.; other details given), then the autoclave was heated at 175degreesC for 60 hours with stirring. The supported membrane obtained was washed with water and dried overnight at 60degreesC; it showed zero permeability to nitrogen at an applied pressure of 40 kPa. After calcination for 20 hours at 520degreesC in air (slow temperature rise at 0.5degreesC/min with an air flow rate of 1.8 Nl/h), the membrane showed permeabilities to nitrogen and sulfur hexafluoride of 30.7 x 10-7 and 0.36 x 10-7 (3.04 x 10-7 and 0.087 x 10-7) mol/s/m2/Pa respectively. Values in brackets were obtained with the zeolite precursor composition 1 SiO2 - 0.4 TPAOH - 27.6 H2O.