Abstract: In order to quickly and economically obtain the information of molecular changes occurring in heavy oils during thermal processing, simulated mixtures of heavy oils and their pyrolyzed products were prepared from four model compounds, i.e., naphthalene, tetralin, decalin, and nheptane. FT-IR absorption characteristics and average molecular parameters of the mixtures were correlated; the molecular parameters included the number ratio of methylene to methyl (NCH2/NCH3), hydrogen aromaticity (faH), atomic ratio of hydrogen to carbon of the aromatic sheet (NHar/NCar), etc. Considering heavy coker gas oil (HCGO) as a heavy oil feedstock, HCGO was thermally processed in an autoclave and its gas oil fraction (GOF) was separated into group fractions and analyzed by FTIR. The results show that the simulated mixtures show good linearity between faH and the absorption area ratio of the bands at 2750cm－1～3100cm－1 to 3000cm－1～3100cm－1 (S3000~3100/S2750~3100), and so doesNCH2/NCH3 and absorbance ratio of the bands at 2920cm－1～2960cm－1 (A2920/A2960). These correlations could be useful tools for illustrating molecular changes occurring in heavy oil during thermal processing. As the processing proceeds, the NCH2/N１CH3 of the saturate molecules increases first and then declines, in sharp contrast to that of the aromatic molecules, which shows a consistent decline. As far as faH or NHar/NCar is concerned, however, the aromatic molecules show a rising trend. The chemistry behind these phenomena is also discussed in some detail.