For more than a century, the need for energy has exerted high demand on oil production and led to significant negative impacts on soil and water resources. The aim of our work was to assess such impacts on the ecological functions of oil-contaminated soils in West Siberia of Russia. The total petroleum hydrocarbons (PHC) content in contaminated soils varied between 3.7 and 390 g kg-1 . Although peat had the ability to absorb some PHC, excess oil migrated in soil both downward and laterally. Catalase activity, soil respiration activity (basal respiration [BR], microbial biomass carbon [Cmic], and specific respiration activity [qCO2 ]), and Enchytraeus albidus survival and reproduction rates showed significant negative correlations with PHC concentrations, and thus they can be used as guides for establishing acceptable PHC limits in peat soils. Based on the Logit model, the concentration of PHC in peat soil that corresponds to ~20% reduction on functions (worm reproduction, catalase activity, and basal respiration) is about 40-50 g kg-1 . The concentrations of PHC that will result in 80% functional reductions (i.e., near-complete loss on functional activities) are worm production (177 g kg-1 ), catalase activity (123 g kg-1 ), and basal respiration (311 g kg-1 ). This study provides quantitative understanding of the ecological impact of PHC contamination on peat soils and thus helps to establish science-based guidelines for the protection of ecological functions and services of peatland soils.