Three molecules of 5-(bromoacetyl)salicylate (1) complexed to Fe(III) ion were crosslinked with poly(ethylenimine) (PEI) in DMSO by alkylation of amino groups of PEI with 1, leading to the formation of Fe(Sal)(3)PEI, a water-soluble polymer. Several other derivatives including the immobilized form were also prepared. Examination of the values of log K-f for the PEI derivatives indicated that each Fe(III) binding site in Fe(Sal)(3)PEI contains three salicylate moieties. In addition, the log K-f revealed that the effective molarity (EM) of the salicylate groups contained in the Fe(III) binding site is ca. 1000 M. The high EM value shows that the geometry of the coordination sphere is well conserved during the crosslinkage with PEI of 1 preassembled around Fe(III) ion. In view of the EM value and the pK(a) values of salicylic phenols in apo (Sal)(3)PEI, the metal-free form, the three salicylate groups of each Fe(III) binding site appear to occupy proximal positions leading to effective cooperation in Fe(III) binding. Fast, strong, and selective binding of Fe(III) ion by the binding site comprising three salicylate moieties was demonstrated. In addition, rapid demetalation of the resulting complexes as well as chemical stability of the immobilized chelating agents built on PEI were achieved.