The study of labile humic substances of typical chernozems under different land uses in Kursk oblast with the help of thermal method attests to their complex composition with participation of aliphatic and cyclic structures heterogeneous in terms of their thermal stainability. According to the Z coefficient (the ratio between weight losses in the low-temperature (<400°С) and high-temperature region (>400°С)) equal to 1.10, aliphatic compounds predominate over cyclic benzenoid compounds in the composition of labile humic substances of virgin chernozems. Under the impact of permanent cultivation of winter wheat and corn, the mineralization of the aliphatic part of labile humic substances increases, and the Z coefficient decreases to 0.81–0.96. The most intense mineralization of aliphatic fragments of labile humic substances occurs under the impact of permanent black fallow, where the Z coefficient decreases to 0.76, and inert cyclic components clearly predominate in the composition of labile humic substances. In 23 years after the conversion of the permanent black fallow into the overgrown unmanaged fallow, the composition of the labile humic substances of the typical chernozem has not undergone significant changes, though a tendency for an increase in the role of aliphatic structures has been noted.