According to radiochemical and biostratigraphic studies of the buried peat layer in the Cheremoshnik key section on the East European plain, the first age dating of this peat was obtained and its assignment to the Mikulino interglacial was confirmed. The Th/U isotope age data obtained for the peat horizon allow us to state with confidence that it was formed during the MIS5e. It was found that a variety of signs, including the lithological characteristics of dated layers and climatic indexes, is evidence that the Cheremoshnik site and Borisoglebsk upland occurred in the periglacial zone during MIS3–MIS2.