NOVELTY - Covalently and ionically crosslinked polymers (I) are prepared by crosslinking polymers having acid groups, polymers having sulfinate groups and polymers having tertiary amine groups with polyfunctional haloalkanes or haloaromatics.    USE - Membranes comprising (I) are useful both in electrochemical applications, especially fuel cells (operating with hydrogen or methanol at 0-180 degrees C), electrochemical cells, secondary batteries and electrolysis cells, and in membrane separation processes, e.g. gas separation, pervaporation, perstraction, reverse osmosis, electrodialysis or dialysis.    ADVANTAGE - (I) combine the hydrolytic stability of covalently crosslinked polymers with the flexibility and good water retention of ionically crosslinked polymers.    DETAILED DESCRIPTION - Covalently and ionically crosslinked polymers (I), including blends of (I) and membranes comprising (I), are prepared by crosslinking polymers having acid groups of formula (II), (III) and/or (IV), polymers having sulfinate groups of formula (V) and polymers having tertiary amine groups of formula (VI) with polyfunctional haloalkanes or haloaromatics to form crosslinks of formula (Ia)-(Ic).    SO3M (II)    PO2M2 (III)    COOM (IV)    SO2M (V)    NR2 (VI)    polymer-SO2-Y-polymer (Ia)    M = H, metal or ammonium;    R = alkyl, hydroxyalkyl or aryl, or NR2 is pyridyl or another tertiary N-containing heteroaromatic or heterocyclic group;    X = Hal or OR;    Hal = F, Cl, Br or I;    Y = (CH2)x, arylene, (CH2)x-arylene or CH2-arylene-CH2; and    x = 3-12.    An INDEPENDENT CLAIM is also included for the preparation of (I), comprising: dissolving the polymers in N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone (NMP), dimethyl sulfoxide or sulfolane; adding the crosslinker; homogeneously dispersing the crosslinker in the polymer solution by stirring; filtering and degassing the polymer solution; spreading a thin film of the polymer solution on a substrate (e.g. a glass or metal plate or a woven or non-woven fabric); removing the solvent by heating to 80-130 degrees C and/or applying a vacuum or in a circulating-air dryer; optionally removing the film from the substrate; and treating the film in dilute (1-70%) mineral acid and then deionized water at a temperature between room temperature and 95 degrees C.