The polymer-electrolyte Wilson model [R. Sadeghi, J. Chem. Thermodyn. 37 (2005) 323-3291 which has a molecular thermodynamic framework has been extended to model the vapor-liquid and liquid-solid equilibrium behavior of amino acids and small peptides in aqueous solutions as functions of temperature, ionic strength and amino acid compositions. The utility of the model is demonstrated with a successful representation of the activity coefficients and the solubility of several amino acids in different aqueous solutions and the results are compared with those obtained from the NRTL model. (C) 2007 Elsevier B.V.. All rights reserved.