The MCD spectra of isostructural (GaCoII)-Co-III and (FeCoII)-Co-III complexes were analyzed. It was shown that the exchange interaction strongly affects the contributions of interacting ions to the spectrum of the whole complex. It was demonstrated that the MCD signal measured at low temperatures in (GaCoII)-Co-III dimer comes from Co-II ion while in (FeCoII)-Co-III dimer the corresponding signal is mainly due to the contribution from Fe-III ion. The polarization of the corresponding optical transitions for both complexes was found. (C) 2011 Elsevier B.V. All rights reserved.