Centerline profiles of temperature, soot volume fraction, and hydrocarbon concentrations were measured in a methane/air co-flowing non-premixed dame whose fuel was doped with cyclopentane, cyclopentene, 1-methylcyclopentene, and indene. Soot volume fraction and aromatic species concentrations were significantly increased by the additives, indicating that the presence of C5 ring-containing compounds in fuels has significant implications for formation of soot and aromatic air toxics in combustion devices. Consideration of the order of the increases indicates that they are attributable to direct chemical conversion of the additives to aromatic species. Cyclopentene is most likely converted to benzene by dehydrogenation to cyclopentadienyl, reaction of the cyclopentadienyl with methyl radical to form methylcyclopentadienyl, and dehydrogenation/isomerization of the methylcyclopentadienyl to benzene. Methylcyclopentene is most likely converted to methylcyclopentadiene by dehydrogenation, followed by the same dehydrogenation/isomerization processes, providing an even more efficient route to benzene. Indene undergoes an analogous methyl addition process to form naphthalene.