The solid-liquid phase equilibrium for the ternary system of L-pyroglutamic acid + L-glutamic acid + water was determined experimentally by the isothermal solution saturation method at temperatures of 298.15, 308.15, 318.15, and 328.15 K under normal atmosphere. According to the obtained solubility data, four ternary phase diagrams and one variable-temperature phase diagram were constructed. Two pure solids, L-pyroglutamic acid and L-glutamic acid, were generated in the L-pyroglutamic acid + L-glutamic acid + water system. In the phase diagrams, there were in all three crystallization zones, two cosaturated solubility curves and one cosaturated point. The variable-temperature phase diagram for the ternary system of L-pyroglutamic acid + L-glutamic acid + water could be used for separation and purification L-pyroglutamic acid and L-glutamic acid from its mixture. The data of solid-liquid phase equilibrium were correlated and computed with the nonrandom two-liquid (NRTL) model and the Wilson model. The root mean square deviation (RMSD) value was no more than 2.12 X 10(-3) for the Wilson model and 40.3 x 10(-3) for the NRTL model; the Wilson model provided better correlation results than the NRTL model for the ternary system at studied temperatures. The mutual solubility data and ternary phase diagrams for the system were of profound significance for optimizing the purification procedure of L-pyroglutamic acid and L-glutamic acid.