Measurements of CO<sub>2</sub>, CO, N<sub>2</sub>O and CH<sub>4</sub> mole fractions, O<sub>2</sub>/N<sub>2</sub> ratios and the stable isotopes <sup>13</sup>C and <sup>18</sup>O in CO<sub>2</sub> and CO have been performed in air samples from the highway tunnel Islisberg (Switzerland). The CO : CO<sub>2</sub> ratios, with an average of (4.15 ± 0.34) ppb : ppm, are lower than reported by previous studies, pointing to a reduction in CO emissions from traffic. The <sup>13</sup>C in CO<sub>2</sub> reflects the isotopic composition of the fuel. <sup>18</sup>O in CO<sub>2</sub> is slightly depleted compared to the <sup>18</sup>O in atmospheric O<sub>2</sub>, and shows significant variability. In contrast, the δ<sup>13</sup>C values of CO show that significant fractionation takes place during CO destruction in the catalytic converter. <sup>13</sup>C in CO is enriched by 3 ‰ compared to the <sup>13</sup>C in the fuel burnt, while the <sup>18</sup>O content is similar to that of atmospheric O<sub>2</sub>. We compute a fractionation constant of (−2.7 ± 0.7) ‰ for <sup>13</sup>C during CO destruction. The N<sub>2</sub>O : CO<sub>2</sub> average ratio (1.8 ± 0.2) × 10<sup>−2</sup> ppb : ppm is significantly lower than in past studies, showing a reduction in N<sub>2</sub>O emissions likely related to improvements in the catalytic technology. We also observed small CH<sub>4</sub> emissions, with an average CH<sub>4</sub> : CO<sub>2</sub> ratio of (4.6 ± 0.2) × 10<sup>−2</sup> ppb : ppm. The O<sub>2</sub> : CO<sub>2</sub> ratios of (−1.47 ± 0.01) ppm : ppm are very close to the expected, theoretically calculated values.