Previous research studies have implicated polar organic nitrogen compounds in fuel instability. Twenty-one middle distillate fuels were investigated for their organic nitrogen content and to determine if any specific organic nitrogen compound might be linked to fuel instability. The organic nitrogen compounds were isolated by mild acid extraction followed by silica gel adsorption. Three extracts were obtained from each fuel sample: a basic nitrogen extract in methylene chloride (BNC), a nonbasic nitrogen extract in methylene chloride (NBNC), and a nonbasic nitrogen extract in methanol (NBNC). The major constituents of each extract were determined by high-resolution gas chromatography/mass spectrometry (GC/MS). After the compounds were identified for each fuel, the fuels were grouped by ASTM stability values to determine if there was significantly more or less of one type of organic nitrogen compound present that could cause instability. The results of this study showed that there was not a specific organic nitrogen compound responsible for instability, but probably an imbalance in either the basic or nonbasic organic nitrogen compounds that caused a shift in equilibrium resulting in sediment or gum formation. This is important to the military because military fuels can remain in storage tanks for I or more years. As fuels are drawn from these tanks, the tanks are subsequently topped off with more recently purchased fuels. In many cases, the mixed fuels are not compatible, resulting in sediment and sludge formation.