The present invention relates to a process for preparing selective ion exchangers based on crosslinked vinylaromatic polymers and containing iminoacetic acid units of the general formulas (I) and (II) wherein X represents hydrogen or an alkali metal ion, by reacting a weak-base anion exchanger bearing primary amino groups with monochloro-acetic acid and/or an alkali metal salt of monochloroacetic acid at a temperature of from 70 to 110° C. and a pH of from 7 to 11.