Carbon isotope analyses of peat profiles from the Karkonosze Mts. (SW Poland) and tree-ring cellulose from Wisa river valley in the Kraków region (S Poland) have been carried out. The samples analysed represent approximately the last 1100 years.The 13C profile in peat from the Szrenica peat bog ranges from -26.74 to -21.81 and the 13C value of tree rings range from -27.82 to -21.94 permil. The observed variations in the 13C values of the peat samples and tree rings from Poland in general correlated with each other (Figure 1). This suggests that the 13C value of organic matter in terrestrial conditions is generally controlled by the same environmental factors. On average, over the last millennium (X-XIX century), the 13C value of peat cellulose has been 1.8 isotopically heavier compared to the corresponding tree ring cellulose value.It is not possible to provide a precise calibration of isotope signatures in tree rings, as the assimilation isotope effect depends mostly on local microclimatic conditions and specific species demands. In the region under study, temperature was the dominant factor controlling the 13C value of tree ring cellulose and peat-bog Sphagnum before the XIX century. It is estimated that, in the temperate climate of Poland, the carbon isotope fractionation between living plants and atmospheric carbon dioxide (13Cp-a) for C3 plants is about -0.26/1 °C. This corresponds to 2.1/1000 m of elevation. Since the mid-XIX, in the Wisa valley when the river was regulated and water deficit in the neighbouring areas became common (Trafas, 1975), water availability plays a primary role in isotope fractionation of the vegetation. Since the 1955, just after the `Lenin' steelworks started operation, pollution became the dominant factor controlling the carbon isotope signature of plants. Contamination of the atmosphere by fossil fuel burning from the `Lenin' steelworks increased the 13C value of tree ring cellulose by about 1.3. This was probably caused by an increased concentration of atmospheric pollution (SOx and dust) limiting the ventilation rate of the stomata.