A molecular thermodynamic framework for the representation of the solubilities of several binary systems, amino acids-water (nine systems) and peptides-water (five systems), has been developed and tested. The proposed model takes into account simultaneously the chemical and physical equilibrium. The latter is described by the UNIQUAC model combined with a Debye-Huckel term. Molal activity coefficient data from the literature were used in order to estimate the new UNIQUAC parameters. These parameters, together with solubility data, were the basis for the correlation of the solubility product of the different amino acids considered in this work. With this approach, very satisfactory results have been obtained for the representation and prediction of the solubilities of amino acids in water as a function of both temperature and pH.