Abstract: The authors study the formation of isotope-oxygen composition of a snow mantle of Bratsk and the fresh snow in Baikal region and define the main trajectories of air masses, bringing precipitation to the city of Bratsk during the cold seasons. The research area covers the city of Bratsk and the points in Irkutsk region – the village Khidiakovo (18 km from Irkutsk), the village Kultuk on the southern coast of the lake Baikal, the mountain Snezhnaya of the route Irkutsk–Bolshoye Goloustnoye and the Tunkin valley in the Republic of Buryatia. The authors register the exceeded content of sodium (4-66 times) in the city snow mantle samples compared with the background snow mantle. Though the man-caused origin of sodium in snow is obvious, the natural causes are also possible. It can be brought by air masses, for example, from the Arctic coast. The study is aimed at this hypothesis testing. The research methods include the isotope method (assessment of the content of stable oxygen-18 in snow, both fresh and settled) and the reverse trajectory method, based on the Semi-Lagrangian scheme HYSPLIT for the period from December 2015 till March 2016 with the end points at the altitude of 3000m at 12 UTC. The average ratio of oxygen-18 content (expressed in the δ-ratio as the content of heavy oxygen in relation to the standard) in the snow mantle in Bratsk was -26, 54‰ in March 2015. Inside the snowpack the δ-rates vary from the horizon to the horizon from -21,52 to 28,1‰. It is considered as a relatively “heavy” isotope-oxygen composition. The authors find out that the influence of the Arctic air masses on the isotope composition of the snow mantle in Bratsk is quite insignificant. Of 44 precipitation days for the period from December 2015 till March 2016, 21 case was connected with the western air-mass transport from the Arctic, 5 cases were connected with the moisture coming from the Arctic sector, 15 cases – with the continental source and 3 cases – with the Sea of Okhotsk.