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.