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[Display omitted] • Evolution of asphaltene-adsorbed aromatics is retarded compared to free ones. • Asphaltene-occluded aromatics profiles respond insignificantly to thermal stress. • Distributions of occluded aromatics are probably thermodynamically controlled. • Steric hindrance imposes a kinetic barrier on the reaction of trapped molecules. The thermal evolution of components trapped in matrix of asphaltenes is supposed to be retarded, as compared to the free ones. Studying the geochemical evolution of trapped polycyclic aromatic hydrocarbons (PAHs) may provide insight for the characterization of high-maturity organic matter. The discrepancies in the thermal evolution between free and trapped (asphaltene-adsorbed/occluded) PAHs (phenanthrene, chrysene, pyrene, and their methylated isomers) were studied by thermal maturation (gold tubes, 300 ∼ 400 °C) experiments on a low-maturity solid bitumen from the Sichuan Basin, southwestern China. The results show that the thermal evolution of asphaltene-adsorbed PAHs is retarded compared to that of free ones. This is attributed to slight differences in reaction kinetics due to steric hindrance by the asphaltene structure. The asphaltene matrix appears to act as a reaction inhibitor leading to a retardation of the thermal evolution of the methylphenanthrene index (MPI) in asphaltene-adsorbed hydrocarbons. This could provide a possibility for maturity assessment of high-maturity organic matter. The thermal evolution of asphaltene-occluded PAHs in the experiments resulted in thermodynamically controlled isomer distributions of occluded methylphenanthrenes and methylchrysenes. However, the occluded methylpyrene isomers were hardly affected by thermal stress. They probably retained their original distributions and may serve as source indicators. [ABSTRACT FROM AUTHOR]