The formation of small PAHs with up to three rings has been studied in fuel-rich, nonsooting premixed flames of propene-oxygen-argon and cyclopentene-oxygen-argon at C/O = 0.77 and 50 mbar. Detailed species composition and temperature profiles for these flames had been studied before, using laser-induced fluorescence (LIF) and in situ molecular beam mass spectrometry (MBMS) with electron impact ionization (EI). These previous studies found a significant influence of the fuel structure upon the reaction pathways to benzene as the first aromatic ring. Here, the further molecular reaction sequence is studied with enhanced sensitivity, using MBMS coupled with resonance-enhanced multiphoton ionization (REMPI). Absolute concentrations of benzene were determined using both ionization strategies in order to establish a link to the previous investigations. The ratio of REMPI signals in the two flames for masses from m/z = 78 (benzene) up to approximately m/z = 200 is given and shows a predominant tendency of the cyclopentene flame to form aromatic species. The experimental results are complemented with simulations using two current chemical kinetic reaction models. The model predictions for important species in this mass range are discussed in the light of reaction flux and sensitivity analyses. (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.