TECHNOLOGY FOCUS - POLYMERS - Preparation (Claimed): Preparation of (I) comprises: dissolving the polymers simultaneously or successively in a dipolar aprotic solvent (N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidinone, dimethylsulfoxide or sulfolane); adding the cross-linking agent; homogeneously dispersing the cross-linking agent in the polymer solution by stirring; filtering the polymer solution and degassing; spreading the polymer solution as a thin film on a base (glass plate, metal plate, woven fabric, non-woven fabric or others); removing the solvent by heating to 80-130degreesC and/or by applying low pressure or in a circulating air dryer; the polymer film is then optionally peeling off the base; and curing the polymer film. Preferred Components: (I) is composed of polymer with at least sulfonyl group of formula (SO2M) or polymer with at least sulfonyl group of formula (SO2Me). (I) consists of a polymer comprising SO2M and SO2Me groups. The base polymer having the functional groups or the base polymers having the functional groups of polysulfones, polyphenylene ether, other polymers which can be lithiated (all preferred), polyether sulfones, polyphenylsulfones, polyether ether sulfones, polyether ketones, polyether ether ketones, polydiphenylphenylene ether, polyphenylene sulfide or copolymers containing at least one of these components. The cross-linking agents are alkyl halo compound of formula (Hal-(CH2)x-Hal) or phenylene alkyl halo compound of formula (Hal-CH2-phenylene-CH2-Hal) (where x is 3-12 and Hal is F, Cl, Br or I). The SO2M groups, POM2 groups and/or COM groups of the polymer/polymer (blend) membrane are hydrolyzed to cation exchange groups of formula (SO3Me), PO3Me2 and/or COOMe (where Me is H, Li, Na, K, Cs, other metal cations or ammonium ions) by treating and curing in aqueous alkali (1-50 wt.%), in completely desalted water, in aqueous mineral acid (1-50 wt.%) at room temperature to 95degreesC, where one or more of the curing steps are optionally be omitted. EXAMPLE - Sulfochlorinated polysulfone Udel (RTM: Poly(ether ether sulfone)) and polysulfone sulfonyl lithium were dissolved in N-methylpyrrolidinone. Then alpha, omega-diiodobutane was added to the solution of the cross-linking agents. The reaction mixture was stirred for 15 minutes the solution was filtered and degassed. A tin film of the polymer solution was knife-coated onto a glass plate. The glass plate was placed into a vacuum dried oven and the solvent was removed at 80-130degreesC and a pressure of 700- finally 15 mbar. The polymer film was peeled off the glass plate under water and was hydrolyzed/after treated at hydrochloric acid and in desalted water at 60-90degreesC for 24 hours to give covalently cross linked (composite) polymer or polymer membrane.