In this study, a mechanism for coke formation of methylcyclohexane (MCH) was investigated by sampling products in a batch type reactor during thermal decomposition. Alkyl substituted C-5 ring hydrocarbons, which have a 5-member ring structure with alkyl groups as side chains, were the major products of thermal decomposition of MCH before coke formation (below 465 degrees C). The composition of alkyl substituted C-5 ring hydrocarbons gradually increased without any decrease during the reactions before coke formation. The composition of alkyl substituted C-5 ring hydrocarbons decreased after considerable increase, when coke formation occurred as 0.084 g coke/ml MCH for 10 h at 465 degrees C. Coke was formed via an increase in polycyclic aromatic hydrocarbons (PAHs). Based on this result, it was hypothesized that the abundance of alkyl substituted C-5 ring hydrocarbons caused the formation of PAT-Is and coke. To identify the role of alkyl substituted C-5 ring hydrocarbons on coke formation, thermal decomposition of MCH with 1,2,3,4-tetrahydroquinoline (THQ) was performed. THQ was used as the hydrogen donor, which inhibited coke formation. When coke formation was completely inhibited with 5.0 wt % THQ at 465 degrees C, the composition of all alkyl substituted C-5 ring hydrocarbons decreased compared to the experiment without THQ In addition, PAHs were not completely formed throughout the reaction. These results demonstrated that an abundance of alkyl substituted C-5 ring hydrocarbons caused the formation of PAHs, which were precursors for coke under supercritical conditions.