The kinetically controlled thermal isomerization of the nadimide endo to exo configuration was investigated in the solid state. Experiments were conducted on 4,4＇-methylene dianiline bis-5-norbornene-2,3-dicarboximide, which is the lowest molar mass compound common to two nadimide end-capped thermosets (the well-known commercially available PMR-15 and a soluble oligobenzhydrolimide). WAXS diffractograms of isomerized moieties, recorded at increasing temperatures, lead to the conclusion that the isomerization can occur in the crystalline state, through retro-Diels-Alder and Diels-Alder reactions.