Global use of biodiesel is increasing rapidly. Combustion of biodiesel changes the emissions profile of diesel engines, altering their impact on both urban air pollution and climate. Here, we characterize exhaust emissions from conventional petroleum diesel and three neat biodiesels manufactured from soybean, canola, and yellow grease feedstocks. Exhaust was sampled from a fixed-speed 4.8 kW diesel generator at idle and full loads, and mass emission rates were determined for nitrogen oxides (NO, NO2, and NOx, particulate matter (PM), and elemental, organic, and black carbon (EC, OC, and BC). Additionally, particle size distributions were characterized. Largely consistent with a growing body of data on emissions from biodiesel, biodiesel emissions were cleaner by most metrics than those for conventional diesel. Emissions from the two primary-oil fuels, synthesized from soy and canola feedstocks, were cleaner by most metrics than emissions from diesel, producing approximately 55, 65, and 60% less PM, EC, and OC at engine idle and 40, 20, and 15% less at engine load. In addition, while primary-oil NOx emissions were 5% higher than diesel emissions at engine idle, they were more than 30% lower at engine load. Yellow grease emissions of PM, EC, and OC were reduced in comparison to diesel at engine idle by 60, 30, and 20%. However, at engine load, most yellow grease emissions were increased in comparison to diesel, resulting in approximately 5, 60, and 70% more PM, EC, and OC. Organic vapor emissions from primary-oil biodiesels were also lower, and aromatic emissions were much lower compared to diesel. Yellow grease NOx emissions were increased in comparison to diesel by approximately 5% at engine idle and 10% at engine load. Notably, NO2 accounted for a smaller fraction of NOx for all three biodiesels compared to diesel, a difference that may be more important than the somewhat higher NOx emissions in determining the impact of biodiesel on urban ozone formation. Taken together, our results suggest that widespread implementation of primary-oil biodiesels could result in improvements in urban ozone and PM pollution. In addition, by reducing both the mass and the EC content of those particles, primary-oil biodiesels may reduce anthropogenic climate forcing.