In the conversion of or heavy oil into valuable fuels by thermal processes, one of the main problems is the formation of soft coke-like substances that call cause equipment. fouling and catalyst deactivation. Asphaltenes are present in large quantity in heavy crude oil and are known to be coke precursors. The objective of the present study was to obtain chemical structure information about the asphaltene molecules when they undergo a thermal treatment and to investigate the mechanism of coke formation, Asphaltenes were separated from two industrial thermal treated heavy Crude oils and characterized by elemental, analysis NMR, FTIR, and FT-ICR MS. Analytical data were then compared with that obtained from asphaltene samples after thermal treatment at 400 degrees C. From complementary kind comparative use of the different analytical techniques, we demonstration that a asphaltenes thermally treated at 400 degrees C tend to aromatize to form coke precursors, We found that the species rich in saturated rings and/or alkyl chains are less stable than the one containing aromatic rings; the main reaction in thermal treatment is the intramolecular cyclization/aromatization and not the cleavage of residual aliphatic chains. Moreover, asphaltene classes containing sulfur atoms present a lower stability than the other species,