The chemical composition and mineral inclusions in peats and the underlying soils of the eutrophic Obskoye fen (Western Siberia, Tomsk Region) are studied. The basic pattern of a change in the mineralogical and geochemical conditions at depth consists in domination of hydroxides on top of the peat deposit and sulfides at the bottom. The geochemical reduction barrier is formed at the bottom of the peat deposit. Its functioning is related to increased concentrations of U and Hg. The markedly pronounced influence exerted by sewage discharge is associated with an increase in the Na concentration in acid extracts from peats and additional accumulation of the mixture of suspended, colloidal, and dissolved forms of some metals in the boggy environment.