The results of oxygen and carbon stable isotope investigations of eight Eemian lacustrine sediments from Poland are presented. The values of δ 18 O changed from ca. −11 to −1‰, and δ 13 C values oscillated between −3.5 and +7.0‰. The isotopic record correlated well with pollen, cladoceran, and diatom data. The evolution of the palaeolakes in Poland was reconstructed. The palaeolakes originated during the final phase of the Wartanian (Late Saalian) Glaciation. The δ 18 O and δ 13 C values of that time reflect the input of detrital carbonates into the basins. The boundary between the glaciation and the interglacial period is expressed by a significant decrease in δ 18 O and δ 13 C values. During the early Eemian, a positive trend in δ 18 O values confirms the gradual climatic changes. The Eemian optimum is characterized by constant δ 18 O and δ 13 C values. During the Early Vistulian (Weichselian) Glaciation, the palaeolake declined. The varying δ 18 O values likely reflect frequent changes in water balance between precipitation and evaporation associated with an influence of marine circulation. The fluctuations of the isotopic curves in the upper parts of the successions (the post-optimum) were caused by a shallowing of the basin by infilling with sediments. The observed shifts in the isotopic curves are due to the proximity of the Baltic Sea and earlier strong oceanic influences.