Concentration profiles of stable hydrocarbons were measured along the centerline of several axisymmetric co-flowing methane/air non-premixed flames whose fuels were doped with 2500 to 1900 ppm of benzene, toluene, ethylbenzene, styrene, and phenylacetylene. The results indicate that the H-abstraction/C2H2-addition mechanism proposed by Frenklach and co-workers was responsible for the naphthalene formed in the undoped, benzene-, styrene-, and phenylacetylene-doped flames. However, a second source of naphthalene, possibly a reaction between benzyl and propargyl radicals, was important in the toluene-and ethylbenzene-doped flames. Soot volume fractions were also measured in each flame; the maximum values correlated well with the maximum naphthalene concentrations, indicating that naphthalene formation is the critical soot formation step for these fuel mixtures containing one-ring compounds.