The electron paramagnetic resonance spectra of Cu2+ doped diaquabis(nicotinamide)bis(o-sulfobenzimidato-N)-cadmium(II) (hereafter, CdNAS) single crystals which were taken at room temperature are discussed. It was found from the analysis of the EPR data that the Cu2+ ions substitute for magnetically inequivalent Cd2+ ions. Two magnetically inequivalent Cu2+ sites were observed. The principal values of the g and the hyperfine tensors were determined. The ground state wave functions of the unpaired electron of Cu2+ ions in two sites were constructed and type of the distortion was determined. (C) 2009 Elsevier B.V. All rights reserved.