A method for peak shape analysis and deconvolution of overlapping endotherms in differential scanning calorimetry (DSC) data has been previously reported. In this study the method is applied to binary mixtures of, (i) (1S,2S)-(+)- and (1R,2R)-(-)-PSI-ephedrine HCl, and (ii) (1S,2S)-(+)-PSI-ephedrine HCl and its diastereomer, (1S,2R)-(+)-ephedrine HCl. Phase diagrams based on enthalpies of fusion at the melting point (DELTAH(f)m) as a function of mol% composition are linear. However, the phase diagrams based on melting temperature as a function of mol% composition are non-linear and show deviations from the theoretical Prigogine-Defay and Schroder-van Laar equations. Estimates of eutectic compositions are more definitive from the diagrams based on DELTAH(f)m values. The phase diagram for the PSI-ephedrine HCl enantiomers demonstrates racemic compound formation with eutectic points for the compound and the pure enantiomers at 28.1 +/- 2.5 mol% and 71.9 +/- 2.5 mol%. The DELTAH(f)m phase diagram for mixtures of (1S,2R)-(+)-ephedrine HCl and (1S,2S)-(+)-PSI-ephedrine HCl suggested the formation of a weak complex with a composition close to 60 mol% of the (1S,2S)-(+)-isomer.