Surface and bulk fouling of four Jet-A fuels has been studied at 185 degrees C. Results are reported for two experimental configurations, In the first, fuel is stressed in a single-pass heat exchanger designed to simulate the application of aviation fuel as a heat sink for cooling aircraft component systems. In the second, fuel is stressed as above, cooled, reoxygenated, and then restressed in another pass through the heat exchanger. The second setup is designed to simulate recirculation of stressed fuel to the fuel tanks and its subsequent use for on-board cooling. Total insolubles per unit volume of fuel are found to be the same for the two experimental setups; however, prestressed fuels tend to undergo more rapid surface fouling upon reheating. The origin of this effect is shown to be chemical changes that occur during the initial stressing. Antioxidant depletion and formation of catalytic products increase the rate of autoxidation in three of the four fuels. Also, the presence of bulk insolubles circumvents delays in deposition usually observed in isothermal stressing of neat fuels.