This article addresses the differences in Soot nanostructure based upon formation and growth conditions. Fuel structure effects can be masked or accentuated depending upon both temperature and rate of increase. Low temperature yields an amorphous soot for all fuels studied here, regardless of flow rate. High temperature yields different results depending upon the rate of increase. A rapid increase as realized by a high flow rate emphasizes pyrolysis kinetics that favor polyaromatic hydrocarbons (PAHs) with five-membered rings leading to soots with many shells and capsules: a highly curved nanostructure. Slower rates result in a different pyrolysis chemistry leading to graphitic soot, as characterized by extended graphitic segments oriented parallel to each other. Reflecting high thermodynamic stability, PAHs may resist decomposition and yield a less graphitic soot nanostructure using either a fast or a slow temperature increase. In contrast, ethanol produces a highly curved nanostructure using either temperature increase rate. (C) 2003 The Combustion Institute. Published by Elsevier Inc. All rights reserved.