Relevance. Importance of studying water exchange at the boundaries of large mire complex in Western Siberia to predict mire formation of the territory and migration of substances in the conditions of climate change. Aim. To assess variation in water and peat chemical composition in the Great Vasyugan Mire marginal area as an indicator of water exchange trends, mineral and organic substances release in climate change conditions. Objects. Nоrtheastern part оf the Great Vasyugan Mire clоse tо Pоlynyanka village, Bakchar district, Tоmsk regiоn. Methods. Laboratory analysis of the water chemical composition was carried out by titrimetry, spectrophotometry, and flame photometry at the Laboratory and Analytical Center of the Siberian Research Institute of Agriculture and Peat. Statistical analysis of the data was carried out using cluster analysis and principal component analysis. Results. Studies showed an increase in the concentration of Са²⁺, Mg²⁺, Na⁺, HCO₃^–, Cl^–, Fe_total in water at the swamp forest area, which indicates the activity of mire formation at the boundaries of the Great Vasyugan Mire with the participation of groundwater and decrease in atmospheric water impact. In the upper layers of the peat deposit in key areas in the swamp forest, an increase in the content of К₂О, MgO, N-NH₄⁺ и N-NO₃^– is noted, which is associated with decomposition of plant residues and their mineralization in dry conditions. As a result, high content of dissolved organic carbon is noted in the waters at swamp forest area, which leads to an increase it concentration in river waters. Studies shown that the forest–swamp contact zone of the Great Vasyugan Mire makes a significant contribution to the total volume of dissolved organic carbon removal from the studied area, which is important to take into account in carbon balance calculation.