We present distribution coefficients and crystal purity data for systems containing two of the three near-isomorphic amino acids L-leucine (LEU), L-isoleucine (ILE), and L-valine (VAL) in aqueous solutions. The data validate our earlier observation that the extent of impurity incorporation in amino acid crystals is related to the ratio of the pure-component solubility of the primary solute (or product) to that of the impurity in the same solvent. A simple quantitative relationship between the impurity distribution coefficients in aqueous systems was also obtained. This relationship makes it possible to predict purity data in isomorphic systems in a common solvent.