Technical Abstract: Biodiesel is a renewable, domestically produced fuel that has been shown to reduce particulate, hydrocarbon, and carbon monoxide emissions from diesel engines. Biodiesel produced from certain feedstocks, however, has been shown to cause an increase in nitrogen oxides (NOx), which is of particular concern in urban areas that are subject to strict environmental regulations. There are several pathways proposed that try to account for NOx formation during the combustion process, one of which is the Fenimore mechanism. In the Fenimore mechanism, it is postulated that fuel radicals formed during the combustion process react with nitrogen from the air to form NOx. We proposed that if these radical reactions could be terminated, NOx production from biodiesel combustion would decrease. To test this hypothesis, we investigated the ability of antioxidants, which terminate these radical reactions, to reduce NOx levels in biodiesel exhaust. Several antioxidants added to a 20% soy biodiesel/80% diesel fuel blend (B20) at a concentration of 1000 ppm were screened using a small, minimally instrumented diesel engine to test their ability to reduce NOx emissions. The engine used for these studies was a single cylinder, direct injection, air-cooled, naturally aspirated Yanmar engine. The NO and NO2 in the exhaust stream were quantified using electrochemical sensors, and differences in NOx emissions from the combustion of B20 with and without antioxidant were compared. The addition of butylated hydroxyanisole or butylated hydroxytoluene reduced NOx emissions, but the other antioxidants tested did not have this effect.