This work compared the surface functional group (SFG) types and concentrations and sp(3)/sp(2) hybridization ratios of in-cylinder soot samples generated by a heavy-duty diesel engine when employing n-heptane and a toluene/n-heptane mixture (20% toluene by volume) as the fuels. In-cylinder soot samples were obtained from a total cylinder sampling system, and the SFGs and sp(3)/sp(2) hybridization ratios were analyzed using Fourier transform infrared and X-ray photoelectron spectroscopy. Despite the differences in fuel formulation, both n-heptane and n-heptane/toluene soot exhibited similar trends in terms of changes in the SFGs concentrations and sp(3)/sp(2) hybridization ratios during the combustion process. However, the addition of toluene to the n-heptane was found to increase the concentrations of all SFGs as well as the sp(3)/sp(2) hybridization ratio. The C-OH and C=O group concentrations exhibited a bimodal distribution for both the n-heptane and n-heptane/toluene soot throughout the combustion process, with the concentrations peaking in the premixed and diffusion combustion phases, respectively. In contrast, the relative amounts of aliphatic C-H groups decreased in the premixed combustion phase, increased in the early diffusion combustion phase, and then decreased in the subsequent combustion phase. The sp(3)/sp(2) hybridization ratios obtained from both fuel soot were observed to initially decrease, then to increase before a decrease during the combustion process. There was a definite correlation between the sp(3)/sp(2) hybridization ratio and the relative concentration of aliphatic C-H groups. (C) 2016 Elsevier Ltd. All rights reserved.