Combustion studies in engines have investigated the chemistry leading to the formation in the exhaust of aromatic hydrocarbons from deuterium-labeled isomeric xylenes. These fuels were: ortho-xylene-d(0) and ortho-xylene-d(10) (1:1); para-xylene-d(0) and para-xylene-d(10) (1:1); and meta-xylene-2,4,5,6-d(4). Isotopic distributions within the exhausted hydrocarbons establish the postflame chemistry involved. There is an isotope effect in the consumption of residual fuel in the postflame region. The residual fuel from each experiment exhibits minimal H-D exchange. Toluene is an intermediate in the formation of ethylbenzene, and is produced through X-. atom (X-. = H or D) displacement of methyl radicals from the xylene fuel. Benzene is formed by direct demethylation, but there are other routes. Styrene from o- and p-xylene fuels is formed intramolecularly, probably involving xylylene and methylcycloheptatetraene intermediates. Ethyltoluene is formed by combination of methyl and methylbenzyl radicals.