C1 to C 12 stable hydrocarbons, soot volume fraction, major species, and gas temperature have been measured in a series of methane/air coflowing nonpremixed flames whose fuel was separately doped with 5000 ppm of five heptane isomers. The temperatures, residence times, major species concentrations, and heptane consumption rates were similar in each flame, so the large differences in hydrocarbon product concentrations result from the direct chemical effects of the heptanes. The heptanes increased the maximum soot volume fraction in the order trimethylbutane > dimethypentanes > n-heptane, which is also the order of increasing number of branches. The species measurements indicated that this occurred because the more-branched heptanes produced more propene and butene and less ethylene, and therefore formed more propargyl radical through dehydrogenation or H-abstraction/beta scission pathways. The results are a useful database for testing detailed chemical kinetic mechanisms of fuel decomposition and hydrocarbon growth from large alkanes. (C) 2003 The Combustion Institute. All rights reserved.