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This paper is concerned with the formation of one- and two-ring aromatic species in near atmospheric-pressure opposed-flow diffusion flames of 1,3-butadiene (1,3-C4H6). The chemical structures of two different 1,3-C4H6/Ar–O2/Ar flames were explored using flame-sampling molecular-beam mass spectrometry with both electron and single-photon ionization. We provide mole fraction profiles of 47 components as function of distance from the fuel outlet and compare them to chemically detailed modeling results. To this end, the hierarchically developed model described by Seidel et al. [16] has been updated to accurately comprise the chemistry of 1,3-butadiene. Generally a very good agreement is observed between the experimental and modeling data, allowing for a meaningful reaction path analysis. With regard to the formation of aromatic species up to naphthalene, it was essential to improve the fulvene and the C5 chemistry description in the mechanism. In particular, benzene is found to be formed mainly via fulvene through the reactions of the C4H5 isomers with C2H2. The n-C4H5 radical reacts with CH3 forming 1,3-pentadiene (C5H8), which is subsequently oxidized to form the naphthalene precursor cyclopentadienyl (C5H5). Oxidation of naphthalene is predicted to be a contributor to the formation of phenylacetylene (C8H6), indicating that consumption reactions can be of similar importance as molecular growth reactions.