NOVELTY - A sulfonated and nitrated aryl polymer, for example poly(etheretherketone) (PEEK) can be prepared either by dissolving the polymer in concentrated sulfuric acid at room temperature and treating with concentrated nitric acid, or dissolving in concentrated sulfuric acid, followed by dropwise addition of nitric acid, with increase in reaction temperature to 60degreesC after a specified time.    USE - The membranes are useful in fuel cells as a replacement for the very expensive perfluorinated Nafion-type ion membranes currently used.    ADVANTAGE - The membranes have high chemical and thermal stability, and the reagents used in the process avoid the high price of the previously used trifluoracetic acid anhydride and the explosion risk of ammonium nitrate.    DETAILED DESCRIPTION - A main chain aryl polymer (MCAP), e.g. an aryl polymer comprising the building units R1, R2 and R5-R8 is nitrated by dissolution of the polymer in 90-100% sulfuric acid and addition of 70-100% nitric acid at a temperature low enough to avoid sulfonation, i.e. 0-30degreesC.    R4 = H, CH3 or 4-phenoxylbenzoyl.    R3 = not defined.    INDEPENDENT CLAIMS are included for:    (1) preparation of nitrated and sulfonated MCAP, nitrated as above and sulfonated in sulfuric acid solution at 60-90degreesC;    (2) preparation of nitrated and sulfonated MCAP with the polymer dissolved in 90-100% sulfuric acid, addition of the required amount of 70-100% nitric acid, and selection of the temperature so that nitration and sulfonation are carried out simultaneously or at least in succession;    (3) preparation of an aminated and sulfonated aryl polymer in which the nitro groups are reduced to primary amino groups by current methods;    (4) preparation of polymer membranes, which can be converted by conventional methods to dense or porous flat membranes;    (5) membrane processes, e.g. fuel cells, membrane electrolysis, electrodialysis, diffusion dialysis, pervaporation, gas separation, ultrafiltration, microfiltration, nanofiltration; and    (6) optionally combined polymer blends.