We investigated the carbon isotope composition of vegetation and soils in steppe pastures of Inner Asia (Southern Transbaikalia, Eastern Mongolia, and Inner Mongolia). It is found that the values of carbon isotope composition in plant samples are low (from –25.43 to –27.57 ‰). This indicates that these communities are wholly dominated by plants with photosynthesis type C3. The steppe phytocenoses under investigation are characterized by an insignificant projective cover, a minimal species abundance, a poorly pronounced layering, and an undersized grass stand. As regards the ecological occurrence, xerophytes are dominant in the vegetation. The values of isotope composition of organic matter in the soils of the upper horizons under investigation (0–20 cm) vary from –27.47 to –24.57 ‰; downward the soil horizon its enrichment by 1–2 ‰ is observed. The isotope composition of humus carbon differs from that of vegetable material by 0.74–2.61 ‰; in the main, it becomes heavier due to destruction by heterotrophic microorganisms. It is suggested that the differences in the carbon isotope composition of plant samples from the phytocenoses under investigation are caused primarily by a different geographical location, and by the natural-climatic characteristics of the study areas. The findings can be used as indicators of ecological processes in determining the degree of impact of the anthropogenic factor (pasture loads) on vegetation cover and, in general, to determine the structure and functional state of natural ecosystems.Keywords: carbon isotope composition, carbonates, organic matter, plant communities, species diversity, steppe phytocenoses.