Fuel-rich laminar premixed flat flames of an acetylene/propene (1:1) mixture and of 1,3-pentadiene were investigated at 50 mbar in order to compare their flame chemistries for identical C/H (0.625) and C/O (0.77) ratios; under these conditions, observed differences in the reaction pathways should be related to fuel structure. Concentrations of the most important species for benzene formation were obtained by molecular beam mass spectrometry (MBMS). Temperature was measured with laser-induced fluorescence (LIF) of seeded NO. The burnt gas temperatures in both flames were similar with maximum values of 2250 K and 2100 K, respectively. The data were analyzed with respect to the formation of C, species, in particular to that of benzene as a key species in the soot formation mechanism. As a consequence of different fuel-specific primary decomposition reactions, the two flames show a strikingly different pattern of intermediate compounds, enhancing different possible benzene formation pathways. Relative reaction flows were also calculated from the experimental results which confirm this observation; however, large uncertainties in some important rate coefficients are noted. While the C,H, recombination reaction contributes an important fraction of benzene formed in each flame, the contribution of e.g. C4H5 + C2H2 cannot be overlooked in the 1,3-pentadiene flame, CA being an important pyrolysis product of 1,3-pentadiene. The present results can also be compared to those obtained under similar conditions in pure acetylene and pure propene flames as well as in flames burning further C-5 fuels including I-pentene and cyclopentene. The consistent data sets provided here as part of this series of systematic investigations should be valuable for testing flame models that include the fuel-rich chemistry of higher hydrocarbons. (C) 2003 The Combustion Institute. All rights reserved.