In this paper, a record of the 14C and 13C isotope content of atmospheric CO2 for Gliwice is presented for samples collected on a weekly basis in the years 2011–2013. In addition, measurements were performed on the early and late wood from the annual rings of pine trees from five sites located 3–6 km from the atmospheric CO2 sampling point. The concentration of 14C in CO2 samples from the air was much lower relative to the concentration of this isotope in “clean air,” indicating a pronounce Suess effect, with a mean Δ14C lower by ca. 60‰ than Jungfraujoch data when the 15% of the highest differences are excluded, which leads to the FFCO2 estimate of 5.8%. In winter, the main source of fossil CO2 was fuel combustion, as confirmed by significant correlations with air pollutants. In the vegetation seasons, the Δ14C was highly variable due to biogenic influence and more variable winds. The isotopic results were also affected by an additional significant CO2 source for the Gliwice air, which was a closed mine shaft. The Δ14C and δ13C in tree rings did not record a strong Suess effect in the years 2008–2013 in woodland areas around Gliwice city.