Crystal polymorphism is of tremendous importance in pharmaceuticals, dyes, 2 semiconductors, porous materials, crystal adaptronics, etc. Ascertaining the thermodynamic relationship between different polymorphs of the same compound is crucial for their application. According to the heat of transition rule, polymorphs are in enantiotropic relationship if transition between them occurs bellow their melting points, which is typically established by differential scanning calorimetry (DSC) experiment. In the case of a monotropic relationship, there is no interconversion between polymorphs. Experimentally measured transition temperature by DSC mostly does not correspond to the thermodynamic transition temperature. Namely, the hysteresis between the experimental and thermodynamic transition temperature is due to the kinetic barrier, which includes nucleation and phase propagation barriers that may be considerable near the transition point. Mechanochemical ball milling 8-10 efficiently introduces crystal defects that lead to polymorphic transitions. The polymorphic outcome of mechanochemical reactions is highly sensitive to the reaction conditions in neat grinding (NG), liquid-assisted grinding (LAG), ion and liquid-assisted grinding, and polymer-assisted grinding approaches. It can also be modified by using different milling jars and balls or by avoiding balls completely. Interestingly, Emmerling and Michalchuk have recently shown how thermomechanochemical reactions lowered the polymorphic transition temperature in isonicotinamide and glutaric acid cocrystals by at least 15 °C compared to the corresponding DSC transition temperature. These experiments were conducted by heating milling jars to the desired temperature setpoints, after which isothermal ball milling was monitored in situ by synchrotron powder X-ray diffraction. Motivated by that report, we aimed to directly determine the onset and completion temperatures of polymorphic transitions in thermo-mechanochemical reactions ‡ by in situ synchrotron powder X-ray diffraction and thermal control of milling (Fig. ). Here, we study polymorphism in a diamondoid derivative, 1-adamantyl-1-diamantyl ether (ADE). Diamondoids are thermodynamically stable cage hydrocarbons whose scaffolds mimic the arrangement of carbon atoms in a diamond crystal lattice. Owing to their unique properties, diamondoid derivatives hold immense potential for application in fields ranging from polymers and surface coatings to organocatalysis, drugs, and molecular electronics. In the solid state these molecules frequently exhibit phenomena such as order-disorder phase transitions and polymorphism. Fig. Our setup for direct measurement of polymorphic transitions temperatures in thermo-mechanochemical reactions.