Carbonaceous soot produced in a small diesel engine test facility was investigated with ultra-small-angle X-ray scattering. Three soot samples produced using a reference diesel fuel and the reference fuel plus two oxygenate additives were investigated. The presence of objects at three typical size ranges, i.e., aggregates, primary particles, and subunits. was observed. By Studying soot powders and pellets from pressed powder, a separation of scattering contributions front aggregates and primary particles was possible. The scattering curves of soot from oxygenated diesel show significant differences between samples obtained Under idle and load conditions. Soot from regular diesel fuel did not show Such differences. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.