In this work, we present mass spectra showing organic species with mass-to-charge ratios between 15 and 900 sampled from near-atmospheric pressure, non-premixed, opposed-flow flames of acetylene, ethylene, and propane using an aerosol mass spectrometer with flash vaporization. Near-threshold photoionization was achieved by synchrotron-generated tunable vacuum-ultraviolet (VUV) light. Among the three different fuels, we observed variation in the mass progression, peak intensities, and isomeric content identifiable in photoionization-efficiency curves. The results indicate that different pathways contribute to the molecular growth of soot precursors and that the significance of these mechanisms is likely to depend on the fuel structure and/or flame conditions. Previous work has highlighted thermodynamic propensities for precursor formation; however, our results suggest that kinetic mechanisms play a role in determining the partitioning of soot precursor isomers under the conditions investigated here. Evidence for aliphatic-bridged and oxygenated species was also observed. Such species have been proposed as a possible precursor to particle inception following cluster formation but have never been confirmed. (C) 2013 Elsevier Ltd. All rights reserved.