The mechanism of endo-exo isomerization of nadimide groups was investigated on 4,4’-methylene dianiline bis-5-norbornene-2, 3-dicarboximide (named PDO) which involves three configuration isomers and on a nadimide end-capped oligomer (D1) both corresponding to the lowest molecular weight species of an oligobenzhydrol-imide nadimide end-capped thermoset (IP960). Isomerization kinetic versus temperature and pressure was studied by H-1 nuclear magnetic resonance. The results indicate that isomerization leads to a thermodynamic mixture composed of almost equal amounts of endo and exo configurations. It was also observed that thermal isomerization of PDO was possible below its melting point and that thermal isomerization under reduced pressure produced maleimide derivatives.