Polymorphism is an interesting property of crystalline substances and is of importance in pharmaceutical development. Studies were conducted to identify the most stable of the five polymorphic (one hydrate and four anhydrous) forms of the active pharmaceutical ingredient (API) E3210. Between the different polymorphs, the transformation mechanisms and thermodynamic stability relationships were complex. Forms A and B were further characterized by differential scanning calorimetry (DSC) at various heating rates and by powder X-ray diffractometry (PXRD; isothermal and temperature-controlled). Analysis by DSC revealed that Form A displayed a single and large endothermic melting peak. Form B displayed two small peaks, one endothermic peak and the other exothermic; the endothermic peak was an enantiotropically related reversible transformation from Form B to Form B', and the exothermic peak was a monotropically related thermodynamically irreversible transformation from Form B' to Form A. Thus, Forms B and B' were enantiotropically related polymorphs, and both were monotropically related to Form A. Form A was the most stable polymorphic form and Form B was metastable.