Hydrocarbon emissions from a single cylinder Ricardo Hydra research engine have been analysed by gas chromatography-mass spectrometry for two isotopically labelled cyclohexane fuels: (1) an equimolar mixture of cyclohexane and cyclohexane-d(12), and (2) cyclohexane-1,1,3,3-d(4). Isotopic distributions of cyclohexane, cyclohexene, benzene, 1,3-butadiene and propylene within the exhaust gases have been used to investigate possible mechanistic pathways in the formation of these emission species. Results from the first fuel show that the exhaust cyclohexane consists entirely of unburnt fuel with a [C6D12]/[C6H12] ratio of 1.4, which indicates the presence of a kinetic isotope effect in the consumption of cyclohexane. The isotopic distributions within the other species are indicative of intramolecular decomposition pathways from the cyclohexyl radical. The exhaust benzene appears to be formed by successive dehydrogenations from cyclohexyl rather than by build up from smaller molecular weight species. 1,3-Butadiene is formed by beta-scission of hexenyl species, whilst propylene is formed through an intramolecular rearrangement, probably involving a methylcyclopentyl intermediate. The second fuel shows that intramolecular hydrogen migration in the cyclohexyl radical is of minimal significance prior to its decomposition to cyclohexene. The isotopic distribution within the observed benzene, however, indicates partial scrambling prior to formation. A preliminary experiment using toluene-d(8) fuel indicates that the conditioning fuel or its breakdown products may pray a role in the formation of exhaust hydrocarbons, particularly in samples taken shortly after the fuel changeover, but that the lubricating oil plays no significant role in the formation of these products.