Insolubilization of pentane-soluble fraction from Athabasca bitumen in thermal cracking at 673 K was examined in N2 and H2 atmosphere and in the presence and absence of solvent. Reaction products were analyzed by 13C-nmr and VPO. In the absence of solvent, insolubilization occurred through two types of mechanisms; polymerization and dehydrogenation. The results of the thermal cracking of pentane-solubles in N2 and H2 atmosphere indicated that polymerization was dominant in the initial stages, while dehydrogenation took place predominantly in the later stages for nitrogen atmosphere. It was also observed that in the absence of solvent the main chemical reactions of pentane-solubles were naphthenic ring opening and consequential cracking of alkyl side chains. Whereas in the presence of solvents, 13C-nmr results indicated that aromatization of products without the naphthenic ring opening and gas formation by the cracking of alkyl side chains occurred in the both atmospheres . However, in the presence of tetralin as a solvent, yield of pentane-insolubles indicated that insolubilization of pentane-solubles was restricted, which could be due to the hydrogen donor ability and the dispersion of bitumen molecules in tetralin .