The reactivity and deposition characteristics of a specification jet fuel (Jet A-1) and synthetic jet fuel produced via the Fischer-Tropsch (F-T) process were investigated under supercritical pyrolytic conditions. The latter fuel was comprised solely of straight-chain and branched paraffins. It was found that the F-T fuel was significantly more reactive and more prone to deposit formation than the Jet A-1. These results for the F-T fuel were attributed to the lack of hydrogen-donor capacity and high kinetic chain length. Overall, the decomposition pathways were consistent with free radical pyrolysis at elevated pressures and moderate temperatures. Analytical measurements support the hypothesis that cyclic unsaturates and polycyclic aromatic hydrocarbons (PAH) are formed as intermediates of the deposition process under these reaction conditions. Following initial thermal cracking, the formation of larger PAHs and deposits occur via addition of low molecular weight hydrocarbons and PAH-PAH reactions.