N-(4＇-benzyl)phenyl-1,2,3,6-tetrahydrophthalimide model compound has been synthesized and its thermal reaction mechanism studied. The products of the N-(4＇-benzyl)phenyl-1,2,3,6-tetrahydrophthalimide (THPI) thermal treatment have been analysed by H-1 nuclear magnetic resonance, size exclusion chromatography and liquid secondary ion mass spectrometry. According to quantitative analysis (column chromatography), only 50% of the reaction products are oligomerization products. Competitive isomerization (20%) and disproportionation (5%) reactions were evidenced.