The article focuses on the analysis of the manuscript of Jan Potocki which was founded in the archives of the St. Petersburg Institute of History (Russian Academy of Sciences). It is the translation into French of some parts of the «Chronicon terræ Prussiæ» by Peter of Dusburg. This translation was used by Potocki for his research in the East European history. Notes of translator contained in this manuscript shows that Potocki gave considerations to toponyms in his researches. Exploring the manuscript authors identifed some sources used by Jan Potocki. To a considerable degree his notes were based on the remarks of Christophor Hartknoch, the first editor of «Chronicon terræ Prussiæ». Other source of information was the appendix of the map of Prussia made by historian and cartographer of 16^th century Caspar Hennenberger A drawing attention to toponyms was one of a characteristic features of Potocki's works, in view of sphere of his interest the history of ethnos. Moreover, Potocki offered a specific representation of the Middle Ages considered as a interlink between Antiquity and the Modern time. Potocki underlined the continuity of toponyms. According to Potocki, analysis of toponyms, gives an indication of the settlement borders between particular ethnic groups. Chronicles written at various times allow to reveal the details of continuity and borrowing the place names of some words by different ethnical groups from each other. Potocki used various dictionaries, as well as lists of words that he has made during his travels in order to establish the linguistic trees from ancient to contemporary languages. Jan Potocki considered the Eastern Europe region not from the position of contemporary national borders, but in terms of «Sarmatia». The study of the region, as a whole, without allocation of internal political boundaries could be inspired by the theory of Sarmatism.