In a recent paper, we demonstrated that the exhaust gas recirculation (EGR) enhanced the oxidative reactivity of diesel engine soot. In this paper, we show that simulated EGR, via carbon dioxide (CO2) addition to the intake air to an engine at concentrations of 0, 2, 4, and 8 vol.% and to the oxidizer stream of an ethylene diffusion flame at concentrations of 0, 5, and 10 vol.%, affects the reactivity of the soot in the same manner as actual EGR. Motivated by this fact, post-flame ethylene soot was produced from a co-flow laminar diffusion flame to better understand the mechanism by which the CO2 affects soot reactivity. This objective was accomplished by successfully isolating and examining the thermal, dilution, and chemical effects of the CO2 on soot reactivity. These three effects account for 45%, 35%, and 20% of the total reactivity of soot respectively, with the thermal effect being the most important factor governing the soot reactivity. The results showed that all of these effects account for a measurable increase in soot reactivity. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.