The solubility of an amino acid in an aqueous solution containing another amino acid was measured at 298 K, The measured amino acid pairs were DL-alanine/DL-serine and DL-alanine/DL-valine. The invariant eutectic compositions of these pairs of amino acids were also determined. In addition, a prediction model of solubility for such a system was developed on the basis of solid-liquid equilibria. To represent more accurately the activity coefficient of amino acids and homopeptides by Larsen’s UNIFAC model, new assignments of UNIFAC groups were proposed and their interaction parameters were determined from experimental osmotic coefficient data. Finally, the prediction of solubilities of mixed solid amino acids was made and the results were compared with experimental data.