The problem of natural desertification as related to the global climate warming is a pressing problem in many parts of the world. The anthropogenic factor (e.g., forest cutting) instigates the natural process. The aim of this work is to analyze desertification processes in the forest-steppe zone of Mongolia and identify the features of these processes in soils of the studied region. Soils under forest and steppe associations in the forest-steppe zone of Mongolia were studied in the southwestern part of the Khentei Upland, southern part of the Selenga Upland, and western part of Khangai Upland. The soil properties were investigated under forests, at the contact between forests and steppes (in the areas of forest cutting), and under virgin steppe communities. The former forest soils in the areas of tree felling and propagation of steppe vegetation have an increased humus content (up to 11.8%) and a greater thickness of the humus horizon (by 10-15 cm); the line of effervescence in them is 10-15 cm closer to the surface, and their pH shifts toward more alkaline values. Thus, the propagation of steppe vegetation into the formerly forested areas is marked in the soils by the increased thickness of the humus horizon and higher humus content. At the same time, the distribution of humus in these soils has evident "forest" features, i.e., a sharp decline in the humus content is registered in the deeper horizons. The fractional composition of humus attests to the similarity of humification processes under forest and steppe communities: in all the soils, humic acids predominate over fulvic acids (Cha/Cfa = 1.3-1.5), which points to the fact that, in the past, the entire area was under steppe communities. The development of forest vegetation leads to changes in the soil hydrothermic regime and slows down humification processes. Under the forests, the upper soil horizons are enriched in poorly decomposed organic remains. These results confirm the conclusion about the paragenetic relationship between forest and steppe communities in the forest-steppe zone of Mongolia, which was made by I.A. Bannikova on the basis of geobotanic studies. One of the indices of soil desertification (the development of erosion, dehumification, salinization, and sanding in the surface horizons) is the accumulation of windblown sand in the surface horizons. Our data suggest that this process is active throughout the forest-steppe zone of Mongolia and manifests itself in the soils under both forest and steppe vegetation. Western winds carrying sand particles predominate in the area. The study of soils from the west to the east shows that the texture of their upper horizons becomes finer in the same direction. Thus, in soils of the western and central parts of the Khangai Upland, the fine sand fraction predominates, and the content of silt and clay fractions is low. In the eastern part of the studied area (on western slopes of the Khentei Upland), particles <0.05 mm predominate, with the coarse silt fraction content of 30-50%. To study this process in detail, a series of soil profiles in valleys of the Ider and Tes rivers were investigated. In all of them, the eolian sediment with a thickness from 1-2 to 20-30 cm was registered. The more mobile fine sand fraction accumulates in the soils at the boundary between forest and steppe communities, whereas a coarser and less mobile sand fraction accumulates in the soils under forests that serve as natural barriers for the aerial transport of sand particles. Thus, our data show that the propagation of steppe vegetation and soil desertification take place in the forest-steppe zone of Mongolia. The former process leads to the increase in the humus content and the thickness of the humus horizon. The accumulation of eolian sand transported with western winds into the forest-steppe zone is seen in the upper soil horizons both under forest and steppe communities. The features of these processes are clearly recorded in the soils. At the same time, the soils preserve the features formed under the previous vegetation communities. This study was supported by the Russian Foundation for Basic Research, project nos. 05-04-49098 and 04-04-48197.