The present work focuses on the impacts of engine operating conditions and combustion phasing on diesel soot properties. The study was carried out in a 2.5 L direct injection common-rail turbodiesel engine using an ultra low sulfur diesel fuel (BP15). The study has two objectives. The first objective is to investigate the reactivity difference of the soot generated at four engine modes, spanning conditions of most interest in the engine operating map. The results show that the impact of engine speed at constant torque is more pronounced than the impact of engine torque (equivalence ratio) at constant engine speed. The effect of the engine torque at constant engine speed, especially at higher speed, is not observable in this experiment. The second objective is to investigate the reactivity and nanostructure of soot generated at different combustion phasing by advancing and retarding the fuel start of injection (SOI) timing. Reaction kinetics obtained from thermogravimetric analysis show that the rate constant of the sample for retarding SOI timing (retarded 2 CAD) is 2.3 times that for advancing SOI timing (advanced 2 CAD). In summary, the results presented here provide unique insights into the methodology that should be used when investigating soot samples generated by different fuels: matching operating modes and combustion phasing for different fuels is suggested while collecting soot samples for characterization. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.