Literature data on the mandelic acid phase diagram are reviewed and examined together with new own results in order to clarify the heterogeneous equilibria in the binary system of the mandelic acid enantiomers and the racemate in water. For this purpose the thermal stability and the melting and recrystallization behavior of racemic mandelic acid are studied using TG and DSC. A metastable racemic phase is prepared from the melt and solution and identified by means of DSC, XRPD and IR spectroscopy as a monotropic modification of the stable racemic compound. Problems associated with handling the metastable phase are addressed. Finally, a mandelic acid phase diagram modified by polymorphy of the racemate is suggested. (C) 2003 Elsevier B.V. All rights reserved.