The correlation and prediction of crystal solubility and purity in mixtures of hydrocarbons and amino acids is described, with emphasis on systems in which solid solution formation is the dominant mechanism that leads to impure crystals. The crystal phase exhibits significant nonidealities even when the dominant mechanism that leads to impure crystals is lattice substitution by isomorphic or near-isomorphic compounds. Nonidealities in the liquid phase are also significant and are described using predictive UNIFAQ and COSMO-RS models. Surprisingly, solute-solute interactions in the liquid phase are shown to play a significant role in isomorphic systems.