Process for preparing polymeric, ionic compounds comprising imidazolium groups, wherein an α-dicarbonyl compound, an aldehyde, at least one amino compound having at least two primary amino groups, if appropriate an amino compound having only one primary amino group and a protic acid, where the carbonyl groups of the α-dicarbonyl compound and of the aldehyde may, if appropriate, also be present as hemiacetal, acetal or hemiketal or ketal, are reacted with one another.