We have studied flames doped with diacetylene and vinylacetylene to determine directly by experiment whether C4 + C2 reactions can be important sources of aromatics in combustion systems. Measurements were made of gas temperatures, major species mole fractions, C1 to C12 hydrocarbon mole fractions, and soot volume fractions in methane/air coflowing nonpremixed flames whose fuel was separately doped with diacetylene, vinylacetylene, propadiene, and acetylene. The dopant concentrations in the unreacted fuel were 5000 ppm or less, which was sufficiently low that the dopants did not affect the flame temperatures, flame size and shape, or H-atom concentrations; thus changes in aromatic hydrocarbon concentrations could be directly attributed to reactions of the dopants and their decomposition products. Diacetylene and vinylacetylene produced larger increases in aromatic hydrocarbon concentrations and soot volume fraction than did propadiene or acetylene. Furthermore, the phenylacetylene to benzene mole fraction ratios were higher in the diacetylene-doped and vinylacetylene-doped flames than in the others. These observations and the profiles of individual hydrocarbons indicate that n-C4H3 addition to acetylene and to diacetylene can be important sources of aromatics in flames where a source of n-C4H3 is readily available.