In 2018 the world community marked 140 years since the establishment of diplomatic relations between Russia and Romania. In the past decade, both Romanian and Russian researchers turned to the history of the Russian-Romanian relations. The work involves both archival sources and publicly disclosed reference materials. The Russian State Historical Archive (St. Petersburg) contains quite a significant collection of sources covering the Russian-Romanian relations. Also the results of censuses of 1890 and 1900 were published as a series of tables. In addition, data from other annual reference books were used. The representatives of the diplomatic corps of Romania accredited in St. Petersburg played a vital role in strengthening the Russian-Romanian relations. The access to the documents and materials of the Russian archives and the published reference books has opened up new opportunities in approaching this subject.