Alkylphenolic antioxidants are commonly used as additives to stabilize gasoline, jet, and diesel fuels. The use of antioxidants in diesel, in particular, has become more common because of increased blending of cracked gas oils and blending of biodiesel. Combustion of these phenolic compounds may have unintended consequences. Nitroaromatic compounds are of particular interest because many are listed as known or suspected carcinogens. During the analysis of previously published data, the identity of an unknown peak was confirmed as 2,6-di-tert-butyl-4-nitrophenol (DBNP), an antioxidant derivative. To determine the source of DBNP, particulate matter samples from combustion of conventional diesel fuels spiked with two possible antioxidant precursors were collected and analyzed by electron monochromatormass spectrometry. Results show fuel spiked with either 2,6-di-tert-butylphenol or 4-methyl-2,6-di-tert-butylphenol produced higher levels of DBNP than control samples. Because fuels require antioxidant additives to prevent degradation, careful selection of antioxidants is important.