A systematic study of the binary melting point phase diagram and ternary solubility phase diagram of the enantiomeric mandelic acid species in water has been undertaken. The melting point phase diagram of the system (+)-/(-)-mandelic acid was determined by means of thermal analysis. In agreement with previous observations mandelic acid was found to belong to the compound forming systems. The solubilities of three fixed enantiomeric mixtures in water (pure enantiomer, racemic, and eutectic compositions) were measured in the temperature range between 0 degreesC and > 90 degreesC. The results revealed that the solubility is strongly affected by temperature in a medium-temperature range. The ideal solubility curves of the three enantiomeric mixtures were calculated and the activity coefficients derived. An empirical three-step model is suggested to describe these activity coefficients.