C1 to C12 stable hydrocarbons, soot volume fraction, several major species, and gas temperature have been measured in a series of methane/air coflowing nonpremixed flames whose fuel was separately doped with 2000 ppm of cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and benzene. The cyclohexadienes mostly dehydrogenated to benzene, while cyclohexane and cyclohexene mostly decomposed to C2, C3, and C4 hydrocarbons. Addition reactions were the main benzene source in the cyclohexane-doped flame, whereas both addition and dehydrogenation were important benzene sources in the cyclohexene-doped flame. Reaction pathways were identified in each flame from the hydrocarbon product distributions and from calculated rate constants of relevant reactions. Dehydrogenation was a minor pathway in the cyclohexane- and cyclohexene-doped flames because unimolecular dissociation, not H-atom abstraction, consumed the dopants and their conjugate radicals in these nonpremixed flames. The maximum soot volume fraction correlated poorly with the maximum benzene mole fraction in the cyclohexadiene-doped flames, which indicates that benzene formation was not a rate-determining step to soot production in those flames. (C) 2003 The Combustion Institute. Published by Elsevier Inc. All rights reserved.