The use of low levels of biodiesel in diesel fuel is becoming more widespread throughout the world, and yet there is still limited information on the actual impact of low concentration biodiesel blends on NOx emissions. For this purpose, two different methyl ester feedstocks produced from soybean oil and animal tallow were tested at B5 and B10 levels in a 2006 Cummins ISM engine and a 1991 DDC Series 60 engine over the Federal Test Procedure (FTP), the Urban Dynamometer Driving Schedule (UDDS), and the Supplementary Emission Test (SET) cycles. Increases in nitrogen oxide (NOx) emissions were found for the unsaturated soy B5/B10 blends for the 2006 Cummins engine over the FTP and UDDS cycles and for the 1991 DDC engine over different combinations of all three cycles. Unlike the unsaturated soy blends, the higher saturated animal fat-based biodiesel did not show consistent NOx increases, with only the B10-animal blend showing a statistical significant increase for the FTP on the 1991 DDC engine. The differences in NOx emissions between the biodiesel feedstocks were likely due to differences in the degree of unsaturation in the ester. The low level biodiesel blends also showed reductions in particulate matter (PM), total hydrocarbon (THC), and carbon monoxide (CO) emissions, consistent with the trends seen for higher biodiesel blend levels. (C) 2016 Elsevier Ltd. All rights reserved.