The analysis of complex, reactive mixtures in flames remains challenging, but the need to understand the formation mechanisms of pollutants such as polyaromatic hydrocarbons ( PAH) and soot drives innovation in this area. We have used molecular beam sampling and single-photon, tunable vacuum ultraviolet (VUV) ionization mass spectrometry to spatially resolve and identify the isomers of PAH precursors in low-pressure, one-dimensional, laminar H-2/1,3-butadiene/ O-2/Ar flames. We have measured the photoionization efficiency spectra of many species in these flames to obtain their ionization thresholds. By comparison with literature ionization energies, the isomers can be uniquely identified. The approach shows promise as a universal probe of PAHs and their precursors from combustion processes.