In this work the solubility of L-phenylalanine anhydrous and monohydrate forms in pure water, a water + acetone mixture, and a water + ethanol mixture at various temperatures were measured systematically using the gravimetric method as well as predicted using the universal quasichemical (UNIQUAC) model. Generally the solubility increased with the temperature and decreased with an increase in the content of acetone or ethanol in the solvent mixtures. The two forms were found to be enantiotropically related, and the transition temperature was about 308.9 K. Besides, the UNIQUAC model was demonstrated to work well for the prediction of the solubility of this pseudopolymorphic system in water and its binary mixtures with a low content of organic solvent.