In modern gas-turbine aircraft, aviation fuel is routinely used as a coolant to remove waste heat loads from, for example, lubrication and secondary engine systems, in addition to its conventional role as the energy source through combustion. The increase of the fuel temperature activates several liquid-phase reactions involving hydrocarbon molecules, dissolved oxygen, and indigenous heteroatomic (O, N, and S) polar compounds. The result of such reactions is the formation of gums and solid deposits within the fuel lines, which are of concern for the maintenance of gas-turbine engines. In the present paper, the thermal oxidative stability of commercial aviation fuel was investigated by the positive electrospray ionization mass spectrometry (ESI-MS) technique. Several classes of polar species, with different levels of alkylation, were detected in the unstressed jet fuel. The majority of these compounds we:re characterized by a molecular mass within the range of 90-250 Da. Thermally stressed jet fuel, under different experimental conditions, showed the presence of a broad molecular mass band of polar compounds in the range of 250-400 Da. This class of soluble high-molecular-mass compounds was not detectable when the dissolved oxygen was removed from the fuel by nitrogen sparging. Consistent with previous findings in the literature, we postulate that aggregation processes of these species, through polymerization or clustering reactions, could be responsible for the formation and precipitation of insoluble compounds, which ultimately lead to the formation of the oxidative deposits.