The oxidation rate of a soot sample is related to its nanostructure, that is, to the curvature and relative orientations of its constituent molecular layers. Soot with curved or disorganized layers generally will have a higher reactivity than soot with planar, graphitic layers. This study used high-resolution transmission electron microscopy to analyze the soot produced by the combustion of three different fuels, two of which contain oxygen bonded within the fuel molecule, in a modern diesel engine. Results show that increasing fuel oxygenation produces lower in-cylinder and engine-out soot levels, consistent with existing studies of the effects of fuel oxygenation on soot emissions from diesel engines. The intriguing new information is that increasing the level of fuel oxygenation produced soot with less graphitic structure and correspondingly higher reactivity. Hence, diesel fuel oxygenation may help curtail soot emissions by enhancing soot reactivity and oxidation rates as well as by preventing certain fuel carbon atoms from participating in reactions that form soot.