Stable isotopic measurements of atmospheric CO (¹³CO, C¹⁸O) were carried out at three season in a middle size city, Mainz in Germany. CO stable isotopic compositions of car exhaust fueled by gasoline, diesel and natural gas were also measured and showed different isotopic compositions (δ¹⁸O vs. V-SMOW = 22.3 ± 0.3‰, 10.9 ± 0.8‰, 20.1 ± 1.0‰ and δ¹³C vs. V-PDB = -29.9 ± 0.3‰, -22.2 ± 1.1‰, -51.3 ± 1.5‰, respectively). The ¹⁸O/¹⁶O ratios of atmospheric CO in Mainz show a clear linear correlation with the reciprocal concentration of CO. This reveals the contribution of CO emitted from a single pollution source (δ¹⁸O = 20.7 ± 0.5‰ V-SMOW), whose isotopic composition is close to that of CO from a gasoline engine combustion. From this value and car exhaust measurements, about 14 ± 7% of CO in Mainz air is estimated to originate from diesel fuel engine combustion. The oxygen isotopic composition can be used as a good indicator of contamination from car exhausts. On the other hand, the ¹³C/¹²C ratios of CO do not show a clear relationship with the reciprocal concentration of CO.