The liquid-phase oxidation of eight fuels and a series of 1:1 blends has been studied at elevated temperature and pressure to simulate some of the chemical-reaction conditions that cause surface fouling in aircraft fuel lines. The time, t, needed to deplete dissolved O 2 by 50% was determined for each blend and component fuel. No simple relationship was found, i.e., linking t for blends and component fuels, attesting to the complexity of aviation-fuel autoxidation. About one-half of the blends oxidize with t being intermediate between the two components. However, many blends between fuels of low thermal stability and fuels which have been severely hydrotreated are found to exhibit unusually slow oxidation, with t being greater than that measured for either component. This phenomenon is not well understood; possible explanations focus on synergistic effects among natural antioxidants, dissolved metals, and aromatics in the blends. Measurements of surface fouling in blends selected for their unusually slow oxidation indicate that thermal stability of fuels can be improved by blending with a paraffinic/cycloparaffinic solvent. Improvements in thermal stability of 1:1 blends are comparable to those achieved by introducing conventional fuel additives.