The isomerization of terpinolene tetrabromide from the cis-2,4- (diaxial) dibromo isomer (A) to the more stable trans-2,4- (equatorial–axial) compound (B) has been investigated in the solid state. The progress of the solid-state reaction as a function of time was followed by solution NMR analysis, and, more effectively, by micropowder X-ray diffraction of the bulk material in situ. Characterization of the solid-state reaction mixture is facilitated by the existence of X-ray crystal structures of both compounds A (monoclinic) and B (reported previously) in addition to a new triclinic polymorph of compound A reported herein, all three forms being observed by powder X-ray diffraction.