The letter of Muscovy ruler Boris Godunov which is held in the Library of Kórnik, is the earliest historical document that testifies about the relations between the government of Muscovy and Ukrainian Cossacks since the end of the sixteenth century. In the article are reviewed the historical context of such contacts and their long term consequences. At that time, led by Krzysztof Kosynskyi, Cossacks entered into conflict with powers of Commonwealth. Godunov informed Cossacks that his government invites them for military service. It was the first time when Cossacks obtained the opportunity to make a choice between the service to the Polish King or Muscovy Tzar. During the next a half of the century, Ukrainian Cossacks transformed into a powerful military corporation, which influenced on the balance of powers in international relations of the region. Later on, when in the consequence of the war with Poland, Cossacks switched themselves to the side of Muscovy in 1654, this change provided a foundation for the hegemony of Muscovy in East Europe.

This article is devoted to a diplomatic (formal) analysis of 13 documents, including 12 originals issued by Russian tsars between 1576 and 1707, which are kept at the Kórnik Library. Among them, there are two original letters by Tsar Michael of Russia dating to 1634 and 1645 and four documents by Tsar Alexis of Russia from 1645 and 1668–1669. The collection also includes Peter the Great’s mandate of 1707 given to Russian negotiators for talks with representatives of the Sandomierz confederation, two extremely interesting documents (in the form of scrolls) of border-related negotiations dating to 1634 and 1645, as well as a notebook of 40 pages containing the Russian party’s proposals presented to Polish envoys during negotiations in Moscow at the turn of 1671 and 1672. The article is enriched with an analysis of the content of four well-preserved tsar’s seals applied to the documents in question.