Presented are the results from investigating carbon balance in forests of the transboundary Selenga river basin, based on inventory data for the status of forest fund and the key factors (tree felling and forest fires) influencing carbon absorption and emission in forests. Analysis of the forest fund in the Buryat and Mongolian parts of the Selenga river basin revealed distinctive features in the age structure of forests (middle-aged and young tree stands are dominant in Buryatia, and mature and overmature stands in Mongolia), and in the structure of lands of forest fund (an increase in forest-covered area is observed in Buryatia, whereas Mongolia shows the reverse trend). It is established that these differences are influenced by the nature of forest use and by the organization of the forest protection against fires in neighboring countries. The ROBUL technique was used to obtain estimates of carbon budget. A comparative analysis of carbon balance showed that the potential of forests of Buryatia in carbon absorption (sequestration) is by a factor of 3 larger than forests of Mongolia. It is concluded that the amount and sign of carbon budget of forests growing in neighboring territories are determined by the current age structure of forests and by the intensity of destructive disturbances (carbon losses as a result of tree felling, forest fires and death of tree stands), because the carbon absorption by forests is characterized by a relative stability. The contribution from various carbon pools of the forest ecosystem to the total carbon reserves and deposition (absorption) by forests is shown. Results obtained from assessing carbon budget can serve as a basis for developing the forest management and forest use strategy within the framework of transboundary cooperation of neighboring countries in order to minimize the negative effects of climate change and make effective use of its potential benefits.Keywords: forest ecosystem, carbon budget, deposition, carbon pools, climate change, forest management.