Carbonaceous soot particles from a spark-ignition direct-injection (SIDI) engine are characterized by physical and chemical techniques. Physical characterization included aggregate size and morphology, primary particle size and internal nanostructure, each by transmission electron microscopy (TEM) and corresponding image analyses. Chemical characterization included composition and bonding as analyzed by X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared - Attenuated Total Reflectance spectroscopy (FTIR-ATR). Engine operational conditions included lean, rich, high-load cases and an advanced ignition test, each relative to a reference condition. The aggregates formed for the operational modes with less time for in-cylinder mixing appeared to be more compact with the primary soot particles exhibiting a higher level of tortuous nanostructure. XPS analysis indicated considerable organic matter content while FTIR-ATR confirmed that the organic component was not condensed volatiles but instead matrix bound organics. A small (<15%) surface oxygen content was observed by XPS, consistent with FTIR results suggesting that the organic content is largely alkyl hydrocarbons rather than oxygenated species. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.