A thermodynamic model is developed for the correlation and prediction of crystal solubility and purity when lattice substitution by isomorphic or near-isomorphic impurities is the dominant mechanism that leads to impure crystals. The model is based on earlier observation that the purity of amino acid crystals obtained from aqueous solutions is related to the relative solubilities of the product and impurity in the solvent. A simple relationship is developed that allows the prediction of the purity of crystals obtained from dilute solutions containing, both product and impurity species.