Carbon deposition from jet fuel on metal surfaces will create problems for the operation of future aircraft. Two jet fuel samples (Jet A and JP-8) were heated in a glass-lined flow reactor in the presence of metal and nonmetal substrates placed in the fuel path. The solid deposits collected on the substrates were examined using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and by temperature-programmed oxidation (TPO). The nature and amount of carbonaceous deposits from the thermal decomposition of jet fuel were determined to be dependent on the substrate properties and jet fuel composition. In particular, the catalysis of carbon deposition by active metals was evident in deposits obtained on single-metal or metal-alloy substrates. Jet A fuel produced much-smaller quantities of carbonaceous solids on active metal substrates than JP-8 fuel did. This variance is attributed to the differences in hydrocarbon and sulfur compound composition of the two fuels.