Abstract <p>The article considers changes of the paleoenvironment on the western edge of continental Europe by the example of the Nantois section deposits, the formation period of which falls on the Middle Pleistocene cryochron, corresponding to MIS 6 and, possibly, to the transition to MIS 5e. The approach used is based on a set of characteristics of specific carbonaceous (humus) substances contained in the deposits, including its most reflective component: humic acids (HAs). It is shown that the correlation coefficients of the indicator parameters of the composition and properties of modern humic acids in soils obtained by different methods for studying their molecular features are within 0.78–0.90. Based on the changes in the characteristics of the humus component of the deposits, it has been established that the latter were formed in changing conditions, when there were five relative warmings of varying severity on the background of generally cold climate, which caused the appearance of clear signs of pedogenesis and formation of primitive paleosols. The revealed trend towards increasing moisture conditions is manifested in an increase in the proportion of fulvic acids (FAs) relative to HAs and in a decrease in the absolute values of the C_ha : C_fa ratio. Preliminary calculations of the mean long-term air temperatures and of the duration of the biological activity period show that their possible limits were between ‒5.9 and –7.8°C and between 52–80 days, respectively. The terms “humus” and “humus substance system” are used as synonyms in this paper.</p>