TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: Network A is a copolymer of at least two monomers, one with aromatic group(s) and cation exchange group(s) selected from imidazolium group(s) and acid group(s) such as carboxylic (COOH), phosphonic (PO3H) and sulfonic acid (SO3H), preferably a copolymer of (a) a monomer with at least one aromatic ring and two polymerisable functions (FP1), with at least one cation exchange group on at least one of the rings selected from imidazolium and acid groups as above, i.e. COOY, PO3Y and SO3Y, in whichY = H, Li, Na, K or NHR3;R3 = 1-5C alkyl, and (b) a second monomer with at least two polymerisable functions (FP2) which can react with FP1 to form covalent bonds. Network A may be obtained by (i) dimerisation of fluorovinyl ether functions or (ii) reaction of sulfonated diamines with epoxide monomers. Network A shows a crosslinker content of 25-50% and an ion exchange capacity of more than 0.5 (preferably more than 1) mmol/g. Network B is a copolymer of at least two monomers, one of which contains CFn groups in whichn = 1, 2 or 3, preferably a copolymer of (c) a first monomer with CFn groups as above containing at least two polymerisable groupos (FP3) and (d) a second monomer with at least two polymerisable groups (FP4) which can react with FP3 to form covalent bonds. Network B may be obtained by (iii) an alcohol-isocyanate reaction, (iv) radical polymerisation of (meth)acrylates, (v) radical polymerisation of di(meth)acrylates or (vi) reaction of epoxides with amines. Network B has a crosslinker content of 0.1-100%. The IPN contains 50-80 wt.% network A (based on A + B). Preferred Method: The monomer mixture is transferred to a mill before polymerisation.Preferred Membranes: Membranes as above with an ion exchange capacity of at least 1 mmol/g, permeabilities to hydrogen and oxygen of less than 5 barrer respectively, a mechanical strength (by DMA measurements) of more than 100 MPa in the dry state at room temperature (RT), a proton conductivity of at least 0.01 S/cm2 in water at RT and a weight loss of zero after 7 days at 70 degrees C in Fenton's reagent (0.01-M iron sulfate in 30 wt.% hydrogen peroxide). EXAMPLE - A solution of 50 mg AIBN in 0.9 g 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl acrylate and 0.1 g polyethylene glycol dimethacrylate (Mn = 330) was mixed with a solution of 4,4'-diaminobenzophenone-3,3'-disulfonic acid (1.5-mM) and trimethylolpropane triglycidyl ether (0.9-mM) in 2.5 ml dimethylacetamide. The mixture was stirred for 30 minutes under argon at room temperature (RT), blade-coated (100 microns) onto a glass plate, polymerised for 15 hours at 60 degrees C followed by 6 hours at 120 degrees C and then dried for 2 hours at 120 degrees C under vacuum. The polymer membrane was removed from the glass and freed from solvent by immersion in 2 l of ultrapure water for 24 hours at RT. The membrane was then ion-exchanged in 1.5 l of 1-M sulfuric acid (1 hour at 80 degrees C) and rinsed in ultrapure water (first at RT to remove residual acid and then for 1 hour at 80 degrees C). The product had a thickness of 50 microns; its mechanical and chemical stabilities were greater than those of prior-art membranes, while its proton conductivity was at least as good and its gas permeability was much lower.