The structure of an ethylene counterflow diffusion flame doped with 2000 ppm on a molar basis of either jet fuel or two jet fuel Surrogates is Studied under incipient sooting conditions. The doped flames have identical stoichiometric mixture fractions (z(f) = 0.18) and strain rates (a = 92 s (1)), resulting in a well-defined and fixed temperature/time history for all of the flames. Gas samples are extracted from the flame With quartz microprobes for Subsequent GC/MS analysis. Profiles of critical fuel decomposition products and soot precursors, Such as benzene and toluene, are compared. The data for C7-C12 alkanes are consistent with typical decomposition of large alkanes with both surrogates showing good qualitative agreement with jet fuel in their pyrolysis trends. Olefins are formed as the fuel alkanes decompose, with agreement between the surrogates and jet fuel that improves for small alkenes, probably because of an increase in kinetic pathways which makes the specifics of the alkane structure less important. Good agreement between jet fuel and the surrogates is found with respect to critical soot precursors Such as benzene and toluene. Although the six-component Utah/Yale surrogate performs better than the Aachen surrogate, the latter performs adequately and retains the advantage of simplicity, since it consists of only two components. The acetylene profiles present a unique multimodal behavior that can be attributed to acetylene's participation in early stages of formation of soot precursors, Such as benzene and other large pyrolysis products, as well as in the Surface growth of soot particles. (C) 2009 The Combustion institute. Published by Elsevier Inc. All rights reserved.