Metallic fuel additives have been considered for soot emission control over the last few decades. However, the exact mechanisms of soot reduction are poorly understood and still remain controversial. In response to the need for identifying the state of the iron additive in the diffusion flame, laser-induced fluorescence and absorption spectroscopy were performed in a laboratory-scale isooctane diffusion laminar flame seeded with 4000 ppm iron pentacarbonyl as the metallic additive. The results of the spectroscopic techniques reveal that the dominant iron species throughout the primary flame region was Fe atoms, rather than any form of iron oxide. Moreover, elemental iron was observed to diminish through the soot oxidation region. The primary conclusion is that the catalytic effect of Fe atoms and possibly iron oxides enhanced soot oxidation in the burnout regime of the flame, thereby reducing the overall soot emissions. (C) 2016 Elsevier Ltd. All rights reserved.