Thermally unstable jet fuels pose operational problems. To adequately identify such fuels, factors that realistically impact on thermal stability were examined. Evaluation was based on a quantitative method of measuring thermal stability, viz., NRL’s recently developed gravimetric JFTOT. This method gives a quantitative measurement of both the strip deposit and filterables formed. The pertinent factors examined included the individual and interactive effects of soluble copper, MDA (metal deactivator), and aging. Aging was accelerated to simulate field conditions of approximately 6 months storage at ambient temperature and pressure. The results indicate that the individual and interactive effects of copper, MDA, and accelerated aging appear to be fuel dependent. On the basis of the results, the three test fuels examined (one JP-8 and two JP-5s) were categorized as exhibiting very good, typical, and poor thermal stabilities, respectively. for both the very good and poor thermal stability fuels, copper in conjunction with aging did not significantly increase the total thermal deposits of the neat fuels. In contrast, for the typical thermal stability fuel, the combined effect of copper and accelerated aging did increase deposits. Furthermore, the addition of MDA prior to aging of the copper-doped, typical stability fuel significantly counteracted the adverse effect of copper and aging. No such beneficial effect of MDA was observed for the poor stability fuel. These findings focus attention on the compositional differences among fuels and the need to elucidate these differences (physical and chemical) for a better understanding and prediction of the fuels’ performances.