As emission standards become more stringent, many studies have been carried out to understand and reduce the emissions from diesel combustion engines, among which nitric oxide (NO) emissions and soot are known to have the trade-off relation during combustion processes. One aspect of this trade-off is manifested by the role radiation heat transfer plays on post-flame gas temperature, thus affecting NO formation. For example, a decrease in in-cylinder soot decreases radiation heat transfer causing an increase in post-flame gas temperature and partially contributing to the corresponding soot-NO relationship with an increase in NO formation.This topic has re-emerged with the increased use of biodiesel; a potential explanation for the so-called "biodiesel NOx penalty" is biodiesel's inherently reduced in-cylinder soot. In order to assess how much radiation heat transfer may affect NO formation, this study seeks to isolate and control the soot formation of petroleum diesel through the use of barium additive. To further assist the analysis of the experimental results, a two-stage model that is capable of predicting in-cylinder NO formation is used.The experimental results show that soot concentrations decrease with an increase in the barium additive. The expected result of a corresponding increase in NO concentration with the soot concentration, however, could not be statistically observed. The behavior of the NO concentrations, for example, is better explained by the changes in ambient temperature than by changes in soot concentration.