3-Bromo-trans-cinnamic acid (3-BrCA) exists as two crystalline polymorphic forms (designated as beta and gamma phases). A polymorphic phase transformation from the gamma phase to the beta phase has been investigated using ex situ powder X-ray diffraction, in situ high-temperature optical microscopy, and differential scanning calorimetry. The transformation occurs at an observable rate at temperatures above about 100 degreesC. A reverse transformation on subsequently cooling the beta phase is not observed. Thermodynamic aspects of the polymorphic 3-BrCA system are discussed, together with kinetic aspects of the transformation from the gamma phase to the beta phase. The structural properties of the beta phase (reported previously) and the gamma phase (determined in this work from single-crystal X-ray diffraction data) are in accord with the alpha/beta/gamma structural classification of trans-cinnamic acid derivatives. (C) 2000 Academic Press.