The invention relates to a sulfonated polyetherketonketone (sPEKK), which is amorphous and/or which can be obtained by sulfonating amorphous PEKK. This sPEKK can be preferably produced by using a diphenyl ether and a benzenedicarboxylic acid derivate. Particularly preferred is the use of benzenedicarboxylic acid dichloride. The benzenedicarboxylic acid derivative is advantageously provided in the form of a mixture consisting of at least two different benzenedicarboxylic acid derivatives, in particular, an isophthalic acid derivative and a terephthalic acid derivative.