The stable carbon isotope ratios of nonmethane hydrocarbons (NMHC) emitted by traffic related sources are presented. Six sets of samples were collected in the greater Toronto area at locations heavily impacted by engine exhaust, fuel losses and fuel evaporation. Furthermore, two series of measurements were made in a suburban area. The stable isotope ratios of alkanes and arenes in the emission studies are on average −27.7±1.7‰ (relative to Vienna Peedee belemnite, VPDB), fully compatible with the average composition of crude oils. On average alkenes are enriched by 2‰ in 13C relative to alkanes and arenes. The differences between measurements at locations impacted predominantly by specific source types, e.g. tailpipe emissions, fuel evaporation, and losses, seldom are statistically significant and only occasionally exceed 2‰. Ethyne emitted from engine exhaust is enriched in 13C by several tens of per mil relative to other NMHC studied. The ambient measurements at a suburban location in the greater Toronto area showed that the ambient stable carbon isotope ratios are close to the source composition, but not completely identical. On average ambient NMHC are enriched in 13C relative to the average of the sources by a few per mil. Furthermore, for most NMHC the ambient measurements exhibit a higher overall variability than the source compositions. The small, but nevertheless often significant differences between source composition and ambient observations can be explained by the isotope fractionation associated with the reaction of NMHC with OH-radicals; the most important atmospheric loss process for NMHC. Overall the results demonstrate that for the metropolitan region the variability of the stable carbon isotope ratios of NMHC emitted from traffic related sources is small.