Gas temperature, C1 to C12 stable hydrocarbon concentrations, and soot volume fractions were measured in an axisymmetric methane/air coflowing nonpremixed flame whose fuel was doped with one mole per cent allene, propene, and propane. The additives did not significantly alter the temperature field, methane mole fractions, or chain-carrying radical concentrations. However, soot volume fractions were increased, in the order allene > propene > propane. The hydrocarbon species measurements indicated that soot formation increases because the additives undergo reaction sequences that raise the concentrations of the benzene and phenyl radical precursors C3H3, C4H3, and C4H5, and consequently enhance the benzene/phenyl formation rate. Therefore, creation of these precursors and of the first aromatic ring are crucial rate-limiting soot formation steps in methane flames.