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The effect of a ferrous picrate based homogeneous combustion catalyst (FPC), known to improve diesel combustion efficiency, on the morphological and chemical characteristics of diesel soot was studied in detail. Diesel soot samples emitted from a laboratory CI engine fuelled with a commercial diesel, with and without FPC doping, were collected and characterised using a combination of several advanced analytical techniques including a TEM fitted with EELS, an FT-IR, and a solid state 13C NMR, in addition to elemental analysis. Compared to the soot from the reference diesel, the soot particle sizes of both primary soot and aggregates from the FPC treated diesel were consistently smaller and decreased with increasing FPC dosage. Both types of soot showed similar fractal dimensions, indicating that there were no apparent changes in the formation mechanisms of the primary soot particles and their agglomeration processes. Furthermore, the types of the carbon bonds and organic functional groups in the soot were virtually unaffected by FPC, as indicated by the similar degrees of graphitisation and indistinguishable chemical structures in the two types of soot. However, the soot from the FPC treated diesel showed slightly higher C/H and C/O ratios than those from the reference diesel. Based on these observations, it was speculated that FPC enhanced the diesel combustion process, leaving fewer soot precursors, and also promoted the oxidation of soot particles, resulting in smaller sizes of the primary soot and aggregates and reduced the overall soot emissions. © 2013 Elsevier Ltd.