This study is motivated by so-called "Biodiesel NOx Penalty'', which is the often reported increase in nitrogen oxides emissions with biodiesel, relative to petroleum diesel. Biodiesel also reportedly decreases exhaust soot concentrations via a reduction of soot formation during combustion. Soot is a known source of radiation heat transfer during diesel combustion, and can cause a reduction in flame temperature relative to non-sooting conditions. As NO formation is strongly dominated by high combustion temperature, the heat transfer from soot to surroundings can reduce NO formation. The primary objective of this study is to assess if a decrease in soot radiation with biodiesel fuel leads to the increase of NO emission. In the current study, a metallic fuel additive (barium), which is known to effectively reduce soot formation during combustion, is used to control the soot formation of both petroleum diesel and 100% palm olein biodiesel. This study shows that the soot radiation effect might not be a dominant factor on NO formation, thus likely contributes little to biodiesel's increase of NOx emissions. (C) 2014 Elsevier Ltd. All rights reserved.