The formation of carbonaceous deposits on metal surfaces in fuel systems is a major concern for the development of advanced aircraft. This study examines the thermal stability behavior and carbon deposition and morphology of carbon deposits of thermally stressed coal-based jet fuels. These materials consisted of hydrotreated 1:1, 2:1, and 3:1 blends of refined chemical oil (RCO) with light cycle oil (LCO). Reactions were carried out in a thermal stressing reactor on an Inconel 718 surface at a 470 degrees C fuel temperature. Chemical compositions of jet fuel samples were determined using gas chromatography-mass spectrometry. In this work, it was found that jet fuels containing low amounts of sulfur and nitrogen and higher amounts of decalins and saturated cyclics gave lower carbon depositions at 470 degrees C. The quantity of carbon deposits formed was found to be independent of the RCO/LCO ratio. The morphology of carbon deposits and chemical composition were also examined. The RCO/LCO ratio in the blend changed the deposit nature and deposit surface coverage. It was also observed that, if two fuels have very similar chemical compositions but different sulfur and nitrogen concentrations, the carbon deposit surface coverage or deposit growth was different.