Attracted by the Bolshevik revolution of 1917, Montandon went to the Soviet Union in 1919. He is forty-two years old. He was one of the first intellectuals to immerse himself in this revolution.
The International Committee of the Red Cross entrusted him with the task of negotiating and organizing the repatriation by Vladivostok of Austrian prisoners of war held in Siberia. It crosses the country via the Trans-Siberian Railway from Moscow to Sakhalin Island, where the last of the Ainu people live. The mission stops en route at the Buryates' home on Lake Baikal, on the borders of Mongolia. He not only deals with the repatriation of prisoners during these two winters in Russia; he also takes the opportunity to study the morphology of the various peoples he met. As a man of equality, he approved of the Bolshevik revolution, including his political police, the Cheka.