A sulfonated aromatic polyether ketone of the formula (II) [Ar--O--Ar'--CO--Ar'--O--Ar--CO--Ar''--CO--] (II) in which from 1% to 100% of the O-phenylene-CO units are substituted with an SO.sub.3M group and sulfonated and unsulfonated O-phenylene-CO units can be in any desired sequence with respect to one another, the radicals Ar, Ar' and Ar'' independently of one another are substituted or unsubstituted 1,2-, 1,3- or 1,4-phenylene rings, and M, taking into account the ionic valencies, comprises one or more elements selected from the following group: H, NR.sub.4+, where R is H or C.sub.1-C.sub.4-alkyl, or an alkali metal or alkaline earth metal or a metal from subgroup 8, and is preferably H, NR.sub.4.sup.+, Li, Na, K, Ca, Mg, Fe or Pt.