Over the course of one year (2021), we monitored the carbon isotope composition of atmospheric CO2 at three locations in Croatia: the Adriatic port city of Rijeka (Cfa climate) and at two rural sites: Gornje Jelenje (Cfb climate) in the vicinity of a main road and clean-air site Parg (Dfb climate). Carbon isotope composition at all sites shows seasonal variation, ranging from –41.3 to 25.2‰ for Δ14C and from –13.1 to –11.3‰ for δ13C. Rijeka systematically has the lowest and Parg the highest Δ14C, and δ13C at the sites are not statistically different one from another. The Δ14C of leaves of deciduous trees reflect the trend of atmospheric Δ14C. Based on the assumption that the investigated area is under the influence of two main sources of CO2: fossil and natural (sea exchange, biosphere, and undisturbed – clean air atmospheric component) the approximate share of fossil CO2 in total atmospheric CO2 has been estimated for Rijeka (2.1 ± 1.3%) and Gornje Jelenje (1.0 ± 0.9%). Comparison of our results with the data from European CO2 sampling stations indicates strong influence of CO2 from sea and biosphere. Backward trajectories indicate a possibility of Δ14CCO2 contribution from distant EU nuclear power plants, but movement of air masses should be considered in more detail to confirm this.