The fuel in a co-flowing methane/air nonpremixed flame was doped with 7400 ppm of 1,3-buradiene, l-butene, isobutene, and n-butane. Temperature, mole fractions of 24 stable C1 to C12 hydrocarbons, and soot volume fraction were measured along the centerline of the five resulting flames. The additives significantly increased the concentrations of aromatic species and of soot. The specific changes could be reasonably explained by conversion of the additives to aromatic ring precursors, but nor by effects of the additives on temperature and H atom concentrations. Isobutene and 1-butene generated the highest concentrations of C3 products, while 1,3-butadiene produced the highest concentrations of C4 products. However, several pathways readily interconvert C3 and C4 species, such that increases in both C3 and C4 species were observed for almost every additive. Recombination of propargyl radicals (C3H3) appears to be responsible for most aromatic ring formation on the centerline of these flames. Reactions of acetylene with n-C4H5 and/or n-C4H3 may also contribute in the 1,3-butadiene-doped flame. Benzene appears to be in partial equilibrium with acetylene in the upper half of the flames. The kinetic processes that establish this equilibrium are most likely ring fragmentation reactions in one direction and acetylene addition to n-C,H, in the other. The probable source of the n-C4H3 is acetylene addition to C2H, with the strong temperature-dependence of C2H concentrations responsible for limiting this process to the upper half of the flames. Production of one-ring aromatic species is an important rate-limiting step to soot formation in these flames.