Two studies were conducted on the effects of the metal deactivator, N,N＇-disalicylidene-1,2-propanediamine (MDA) on jet fuel thermal stability. Study 1 focused on the effects of MDA on extended duration thermal stability testings ranging from 120 to 152 h. The test fuel was a Jet A, and the effects of MDA were examined with and without the addition of the antioxidant, BHT (2,6-di-tert-butyl-4-methylphenol) as well as with and without added copper. For study 2, the thermal stability test duration was 2.5 h, and the effect of time of addition of MDA, which was also investigated in study 1, was further examined in a test matrix that comprised two JP-5 fuels, two sources of copper, and three concentrations of MDA. Thermal stability was determined using the gravimetric JFTOT. The results of the extended duration testings indicate that even at the relatively high MDA concentrations examined (15 and 35 ppm), MDA exhibited a beneficial effect in the non copper doped commercial (Jet A) and Navy type fuel (Jet A + BHT). The results of the time of MDA addition studies indicate that early addition of MDA is more effective than late addition, for copper-contaminated stored jet fuels.