A high pressure plug flow reactor designed for supercritical hydrocarbon reaction studies was applied to the analysis and fuel fouling aspects of the endothermic fuel methylcyclohexane (MCH). At a reduced pressure of 1.3 (4.5 MPa), a residence-time of 0.9 minutes, and temperatures from 750-344 K, conditions typical of the most extreme aircraft conditions envisioned for the future, gaseous and liquid MCH products were analyzed. At the lower temperatures examined conversion of MCH to Polynuclear Aromatic Hydrocarbons (PAH) was essentially zero. At higher temperatures, above 820 K, significant PAH formation was observed, as was particulate formation, a very important practical result and a trend established for soot formation in gaseous diffusion flames. Operation in the supercritical region revealed varying temperature to be significantly more important than pressure with regard to PAH and particulate formation. Chromatograms of MCH reactor products established that there were hundreds of gaseous and liquid intermediates under both the sub- and supercritical conditions of operation. In gas phase experiments at 1 atm and temperatures around 1100 K in the Princeton Turbulent Flow Reactor, explicit equations for the rate of decay of MCH and another endothermic fuel decalin were determined. Significantly, product compositions of both fuels were found to be very different from those observed by others for the catalytic decomposition of these fuels.