The roles of aliphatic and aromatic chemical species in soot mass growth were studied in a burner-stabilized premixed ethylene-oxygen-argon flame at equivalence ratio phi = 2.5. Temperature, soot size distribution, and volume fraction were measured as a function of distance from the burner surface. The chemical composition of the soot was determined using a novel aerosol mass spectrometric technique, photoionization aerosol mass spectrometry (PIAMS), spatially resolved as a function of height above the burner surface (HAB). At lower HABs, the soot chemical composition was dominated by polycyclic aromatic hydrocarbons (PAHs) containing 16 to 30 carbon atoms. These measurements confirm that during particle inception and initial growth, the increase in particle mass results predominantly from an increase in the amount of PAH mass. Somewhat unexpected, ions corresponding to saturated and/or unsaturated hydrocarbons in the soot increased substantially as HAB increased. At the late stage of soot mass growth, the masses of aliphatic and aromatic components are similar. These observations indicate that for the flame tested, aliphatic compounds make a notable contribution to soot mass growth. The large aliphatic contribution coincides with a liquidlike particle morphology observed by TEM. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.