||We investigated the isotopic composition of CO in the exhaust of individual vehicles. Additionally, the CO 2 isotopes, and the CO:CO 2 , CH 4 :CO 2 and H 2 :CO gas ratios were measured. This was done under idling and revving conditions, and for three vehicles in a full driving cycle on a testbench. The spread in the results, even for a single vehicle, was large: for δ 13 C in CO ∼ −60 to 0‰, for δ 18 O in CO ∼ +10 to +35‰, and for all gas ratios several orders of magnitude. The results show an increase in the spread of isotopic values for CO compared to previous studies, suggesting that increasing complexity of emission control in vehicles might be reflected in the isotopic composition. When including all samples, we find a weighted mean for the δ 13 C and δ 18 O in CO of −28.7 ± 0.5‰ and +24.8 ± 0.3‰ respectively. This result is dominated by cold petrol vehicles. Diesel vehicles behaved as a distinct group, with CO enriched in 13 C and depleted in 18 O, compared to petrol vehicles. For the H 2 :CO ratio of all vehicles, we found a value of 0.71 ± 0.31 ppb:ppb. The CO:CO 2 ratio, with a mean of 19.4 ± 6.8 ppb:ppm, and the CH 4 :CO 2 ratio, with a mean of 0.26 ± 0.05 ppb:ppm, are both higher than recent literature indicates. This is likely because our sampling distribution was biased towards cold vehicles, and therefore towards higher emission situations. The CH 4 :CO 2 ratio was found to behave similarly to the CO:CO 2 ratio, suggesting that the processes affecting CO and CH 4 are similar. The δ 13 C values in CO 2 were close to the expected δ 13 C in fuel, with no significant difference between petrol and diesel vehicles. The δ 18 O values in CO 2 for petrol vehicles covered a range of 20–35‰, similar to the δ 18 O of CO. The δ 18 O values in CO 2 for diesel vehicles were close to the δ 18 O in atmospheric oxygen. A set of polluted atmospheric samples, taken near a highway and inside parking garages, showed an isotopic signature of CO and a H 2 :CO ratio that were similar the high emitters in the individual vehicle measurements, with no significant differences between parking garage and highway samples. This suggests that in both environments, which are dominated by different driving conditions, the CO emissions from high emitters (either a few high emission vehicles, or many vehicles with brief bursts of high emissions) dominate the total traffic emissions.