The reduction of emissions from diesel engines has been a key element in obtaining air quality and greenhouse gas reduction goals. Biodiesel is an important alternative fuel for diesel applications, but there is a tendency for biodiesel to increase nitrogen oxides (NOx) emissions, which remains an issue in nonattainment areas. This study investigated the effect of using low blend level biodiesel fuels and fuel additives on emissions. Emissions from three B5 biodiesel fuels and six B20-soybean oil methyl ester (SME) with additive blends were evaluated as potential biodiesel formulations for California. B5-SME and B5-waste cooking oil methyl ester (WCOME) both showed measurable increases in NOx emissions, while a B5-animal fat methyl ester (AFME) showed a slight reduction or no change in NOx emissions compared to the CARB diesel. The B5-AFME blend also passed the criteria of the CARE diesel emissions equivalent certification test. Of the additives tested, only one provided reductions in NOx emissions for the B20-SME blends, but the reductions were not enough to pass the CARE diesel emissions equivalent certification test at the B20 level. Biodiesel blends generally showed either reductions or no significant changes in particulate matter (PM), total hydrocarbon (THC), and carbon monoxide (CO) emissions. (C) 2014 Elsevier B.V. All rights reserved.