The present invention relates to a method for producing iron oxide/iron oxyhydroxide-containing carboxyl-bearing ion exchangers, which is characterized in that a) a bead-type carboxyl-containing ion exchanger is contacted in aqueous suspension with iron(III) salts or a′) an aminomethylated crosslinked polystyrene bead polymer is contacted in aqueous suspension with iron(III) salts and with chloroacetic acid and b) the suspensions obtained from stages a) or a′) are adjusted to pHs in the range from 3 to 14 by adding alkali metal hydroxides or alkaline earth metal hydroxides and the resultant iron oxide/iron oxyhydroxide-containing ion exchangers are isolated by known methods, to the ion exchangers themselves their, and also to their use for the adsorption of heavy metals, in particular arsenic.