Thermal decomposition of n-butylbenzene and n-butylcyclohexane was studied under near-critical and supercritical conditions in relation to future jet fuel thermal stability problems. The reactions of n-butylbenzene and n-butylcyclohexane can be explained by free-radical mechanisms, dominated by side-chain cracking. The major liquid products from n-butylbenzene were styrene and toluene. Toluene was the major product in the far supercritical region while styrene was the dominant product in the low-pressure subcritical region. The main liquid products from n-butyldyclohexane were 1-methylcyclohexene and cyclohexane._ The 1-methylcyclohexene was a secondary product which was derived from methylenecyclohexane. This conversion was favored at high pressures. High pressures under supercritical conditions promoted radical addition reactions, leading to the formation of some high-molecular-weight compounds which were not observed under low-pressure conditions. The kinetic data obtained for the thermal decomposition of n-butylbenzene and n-butylcyclohexane are in good agreement with the values reported in the literature.