In the present paper, the thermal stability behavior and amount of carbon deposit from thermally stressed jet fuel candidates were examined as functions of fuel treatment and reaction temperatures. The jet fuel candidates were refined chemical oil, a byproduct of coke manufacture saturated by high-pressure hydrogenation (HP-Sat-RCO); light cycle oil, a petroleum product saturated by high-pressure hydrogenation (HP-Sat-LCO); 1:1 blends of these two materials; two (1:1 vol) RCO/LCO blends saturated by low-pressure hydrogenation; and JP-8. They were tested in a flow reactor on an Inconel 718 surface to assess their thermal stability. All fuels showed marked improvements in stability when the fuels were alumina-treated. Alumina treatment decreased the amount of carbon deposit even if carbon deposition was already very low for the untreated fuel. Chemical compositions of jet fuel samples and proton distributions were determined using gas chromatography-mass spectrometry (GC/MS) and nuclear magnetic resonance (NMR). These results showed that alumina treatment did not change chemical composition and proton distribution significantly. Nitrogen-containing compounds were totally removed by alumina treatment, whereas sulfur- and oxygen-containing compounds were only partially removed. Alumina treatment essentially removed peroxides. When peroxides, nitrogen-bearing compounds, and some sulfur-containing compounds were removed from the fuels, the carbon deposition was retarded.