The flux and isotopic composition of soil CO₂ has been monitored at three sites located in the southern Poland, during the time period: January 1998 - December 2000. The sites represent typical ecosystems appearing in central Europe: mixed forest, cultivated agricultural field and grassland. To monitor the flux and isotopic composition of soil CO₂, the method based on the inverted cup principle was used. The flux of soil CO₂ reveals distinct seasonal fluctuations, with maximum values up to ca. 20 mmol·m^-2 h^-1 during summer and around ten times lower values during winter. Also significant differences among the monitored sites were detected, the CO₂ flux being the highest for the mixed forest site and ca. two times lower for the grassland site. The ¹³C content of the soil CO₂ flux is nearly constant throughout the year, with d¹³C values essentially reflecting the isotopic composition of the soil organic matter and the vegetation type. The ¹⁸O content of the soil CO₂ flux shows a remarkable seasonality, with distinctly less negative d¹⁸O values recorded during summer. This seasonality is highly reduced in the CO₂ sampled from different depths of the soil. The radiocarbon content of the CO₂ flux turned out to be non-distinguishable, within the uncertainty of the measurements, from current ¹⁴C levels in atmospheric carbon dioxide. However, significant reduction of the radiocarbon content was measured in the soil CO₂ sampled at 80 cm, when compared to the uppermost layer, reflecting increasing age of soil organic matter with depth.