Observations of 14C in atmospheric CO2 at four different sites in central Europe, Heidelberg, Westerland, Schauinsland and Jungfraujoch have enabled us to determine individual fossil-fuel contributions to atmospheric CO2 concentration. The data clearly show a decrease of fossil-fuel CO2 with distance from anthropogenic source regions. At Heidelberg during winter we observe 14C/12C ratios up to 10% lower than at the clean air mountain station Jungfraujoch in the Swiss Alps, corresponding to an anthropogenic CO2 contamination level of ca 10% at the Heidelberg site. The Schauinsland and Westerland winter fossil-fuel CO2 concentrations are only ca 1.5 and 2% of the mean concentration, respectively. Our results indicate a strong seasonality in the European fossil-fuel CO2 source with ca 50% lower CO2 emissions during summer if compared to winter fossil-fuel CO2 release. This effect may significantly contribute (by 1–2 ppm) to the observed annual cycle of atmospheric CO2 concentration in northern mid-latitudes.