The thermodynamic behavior of DNA solutions in the presence of salts is studied using a new phenomenological model of Helmholtz free energy of the system. The activity coefficient of the DNA-salt solution system is calculated and its asymptotic behavior suggests the formation of gel precipitate. Phase instability analysis of the derived free energy predicts multivalent salt concentration at which DNA precipitates. However the model is incapable to predict the redissolution of DNA at higher salt concentration. (c) 2006 Elsevier B.V. All rights reserved.