Several anions were found to penetrate into a strongly acidic cation-exchange resin by the aid of complexation with Fe3+ ion to give dimeric species, such as [(-S)(2)Fe(A)Fe(S-)(2)] (-S: benzenesuffonate bound to polymer network; A(2-): HPO42-, HPO32-), and species having a composition of (-S):Fe:A = 2:1:1 (A(-): OH-, H2PO2-).