Future high-speed aircraft will be challenged to meet their onboard cooling requirements, because of weight limitations and higher heating loads. Therefore, the fuel will be required to serve as both propellant and coolant, and, thus, the fuel will need to be thermally stable. An advanced thermally stable jet-fuel formulation, called "JP-900", has been under development for such applications. Six formulations of potential JP-900 basestocks were tested in a flow reactor to assess their thermal stability and resultant deposit morphology. Of the six fuels that were tested, three fuels consisted largely of hydroaromatic and aromatic compounds, and the other three fuels consisted mainly of cyclic compounds. The fuels were made from refined chemical oil (RCO), a product of coke manufacture, light cycle oil (LCO), a petroleum product, or blends thereof, and were hydrotreated to different extents. All six fuels showed marked improvements in stability when compared to the conventional military jet fuel JP-8. The fuels that consisted largely of hydroaromatic compounds were stable at high temperatures but produced large amounts of deposits in the autoxidative regime, whereas the fuels that consisted of cycloalkanes were more stable in both the autoxidative and pyrolytic regimes than their less-saturated counterparts. Basestocks derived from RCO were more stable in the pyrolytic regime than those derived from LCO. This observation was attributed to their differences in gross fuel composition. Removal of dissolved oxygen via nitrogen sparging increased both the autoxidative and pyrolytic stability of all six fuels. There were also morphological differences in the high-temperature deposits formed by the sparged and nonsparged fuels, which indicated differences in the deposition mechanism when dissolved oxygen was present in the starting fuel.