The formation of pyrolytic solid deposit, or coke, in the fuel line can be detrimental to the operation of high-speed aircraft., Yet, the formation of coke from the fuel has not been well characterized. The present study has investigated the relationship between the formation of aromatic compounds and solid deposition for three candidates for high-thermal-stability jet fuels at 482 degreesC (900 degreesF) with stressing periods up to 2 h. The fuels include one coal-derived (JP-8C), one paraffinic petroleum-derived (JP-8P), and one naphthenic petroleum-derived (DA/HT LCO). The DA/HT LCO, an extensiirely hydrotreated light cycle oil where virtually all aromatics have been hydrogenated to cycloalkanes, suppressed the solid deposition to a greater extent than that of the more paraffinic petroleum-derived JP-8P and showed a comparable low solid deposition to that of the coal-derived fuel JP-8C. Both GC/MS and solution-state C-13 NMR analysis on the stressed fuels confirmed that the paraffinic content is most likely to crack under thermal stress, while cycloalkane structures are more thermally stable. Solution-state C-13 NMR and HPLC investigations of the overall structure of the stressed Liquids indicate that the solid deposition is a function of the rise in the aromatic content and also the amount and rate of development of the nonprotonated aromatic carbons, giving mostly 2 to 4 rings aromatics. Furthermore, solid-state C-13 NMR was used to follow the development of the aromatic structure in the corresponding solid deposit as a function of the buildup of aromatic compounds in the stressed Liquid fuel.