The formation of two-ring aromatics in hexylbenzene pyrolysis is used in this work as a model system toward understanding the underlying chemistry of coke formation during crude oil upgrading processes. In this work, batch reactor experiments were performed at 55 bar and 450 degrees C to study the pyrolysis of hexylbenzene utilizing two dimensional gas chromatography mass spectrometry (GCxGC-qMS) as an analysis tool. This work finds many different classes of aromatic species with more than one ring, including bridged two-ring aromatics, nonfully aromatized fused two-ring aromatics, fully aromatized fused two-ring aromatics, and >2-ring species. A kinetic model detailed with elementary chemical reactions is constructed using the reaction mechanism generator, with thermodynamic and kinetic parameters calculated by the CBS-QB3 quantum chemistry method. The results of the generated model were compared to batch reactor experiments. For many compounds, the predicted selectivities agree with the data within a factor of 2. The formation pathways of previously less-understood species were clarified based on evidence of intermediates predicted by the model and measured in this work's experiments. Despite these advances, there are larger model-versus-data discrepancies in the selectivities of some species, and also, the overall conversion rate is underestimated, indicating the need for future work to further improve thermochemical and kinetics data of alkylaromatics.