Peat soils of the taiga zone of West Siberia have historically been relatively poorly studied. In the diagnostics of peat soils, the question of the belonging of the sphagnum litter horizon to the soil profile, as well as the identification of its lower boundary, remains unresolved. In the WRB and Russian Soil Classification, sphagnum litter is considered as a vegetation cover, while in the Soviet classification it is considered as an integral part of the soil profile. The last point of view is also shared by the majority of Russian researchers. Using the material obtained in the study of peat soils in the basin of the river Kazym (subzone of the northern taiga, West Siberia), a comparative characteristic of the sphagnum litter horizon (0–20 cm) and the underlying peat horizon (20–50 cm) was carried out using three parameters: the botanical composition of peat, the degree of peat decomposition, and the color of the soil. All soils are differentiated into the litter horizon and the peat horizon by at least one parameter (5% of the profiles), but in 71% of cases, by three at once. The degree of profile differentiation into two horizons tends to increase in a series of soils formed, respectively, in oligotrophic pine-shrub-sphagnum, oligotrophic complex ridge-hollow, and mesotrophic biogeocenoses. In the overwhelming majority of oligotrophic peat soils, the transition from the litter horizon to the peat horizon is gradual, which does not allow a reproducible assessment of the boundary position in the soil profile. It is proposed to establish a fixed border of the litter horizon at 20 cm from the surface of the bog, referring it to the surface horizon of peat soil.