The influence of the crosslinking degree and monomer dilution with a porogenic agent on the binding of copper(II) by four amidoxime chelating resins was studied by EPR. The amidoxime resins were characterized by FTIR and surface area measurements, and were loaded with Cu(II). Room temperature EPR spectra of the Cu(II)-loaded resins were best simulated with two Cu(II) species, namely Species 1, corresponding to isolated Cu(II) sites, and Species 2, whose broad lines where assigned to clustering of Cu(II) centers. All the spectra also presented a pattern of ligand hyperfine lines (a(N) = 1.4 mT) corresponding to two magnetically equivalent nitrogen nuclei bonded to the Cu(II) center. The concentration and distribution of amidoxime groups in the polymer matrix influenced the environment of Cu(II) ions. The formation of Cu(II) clusters was improved by two factors: high pore diameter and high amidoxime group content of the chelating resins. (C) 2011 Elsevier Ltd. All rights reserved.