Calorimetric and spectroscopic studies on the interaction of Calf thymus DNA with divalent metal ions have been carried out in order to investigate how different ionic strengths as well as different divalent cations affect the thermal stability of the DNA double helix. The thermodynamic parameters of thermal denaturation of ds-DNA have been determined from solutions containing Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and Cd2+ ions in different concentrations. The results obtained indicated that the nature of the interactions of the metal ions with the DNA molecule depends on metals ion concentration. At low metal concentrations there are no significant changes in the melting temperature value. However at high metal concentrations a decrease in melting temperature was observed, showing that the presence of high divalent cation concentration decreases the double helix stability. Further, it is also shown that the cations tested have significantly different interactions with DNA, even at the same concentration. This reveals clearly that not only the ionic strength is important in DNA stability, but that the changes observed in stability and thermal profile depend largely on the metal used. (C) 2009 Elsevier B.V. All rights reserved.