The organic emissions from a diesel engine generally comprise a substantial proportion of hydrocarbon components of fuel-surviving combustion. Accompanying these in the engine exhaust are other components, often identical in structure to the surviving species but;which have been produced pyrosynthetically from partially burned fuel. This paper reports on a series of radiochemical spike experiments designed to unequivocally elucidate the degree of survival and the extent of pyrosynthesis of the important fuel component naphthalene under one particular speed and load condition. Emissions were examined after duplicate experiments in which diesel fuel spiked with [C-14]naphthalene was combusted in a Perkins Prima DI diesel engine. Diesel exhaust samples were collected using the total exhaust solvent scrubbing apparatus (TESSA) sampling system. Analysis of the exhaust samples was performed by radio-high-performance liquid chromatographic techniques developed for this research. The major radioactive species in the exhaust was identified as [C-14]naphthalene, which had survived combustion. The survival represented 0.48% of the original activity. The initial specific activity of the radiochemical in the fuel, for the duplicate experiments, was 1370.60 and 1373.84 mCi/mmol, respectively. Corresponding specific activities of [C-14]naphthalene in the exhaust emissions were 327.5 and 317.5 mCi/mmol, respectively. From this, it may be concluded that the contribution to naphthalene recovered in the, emissions from naphthalene-surviving combustion was 23.8% while other sources of naphthalene, presumably pyrosynthetic in nature, represented the major proportion (76.2%) of the recovered naphthalene. The sources of the pyrosynthesized naphthalene in the emissions were investigated in a further series of experiments involving both [C-14] labeling and a nonlabeled fuel enrichment technique. Fuel spiked with [C-14]-2-methylnaphthalene was combusted, and in the exhaust extracts were recovered both radiolabeled naphthalene (0.036% of the original [C-14]-2-methylnaphthalene) and radiolabeled 2-methylnaphthalene (0.45% of the original [C-14]-2-methylnaphthalene) that had survived combustion. This experiment showed unequivocally that a-methylnaphthalene was converted to naphthalene in the combustion chamber. Enrichment experiments in which both 1- and 2-methylnaphthalene were added to the fuel just prior to combustion confirmed that demethylation of both species produced naphthalene in small yields (1.79% and 6.80%, respectively, of the naphthalene present in emissions). Reaction mechanisms are suggested for the formation of naphthalene from methylnaphthalene. These include oxidation of the side chain and hydrogenalytic demethylation.