The catalogue was prepared within the framework of the research project Court Books of the Grand Duchy of Lithuania (GDL): Digitization and Database, realized by Vilnius University Library in 2011–2014 (VUL, No. LIT–5–19). The paper deals with the previously unexplored Trakai Castle Court Acts of 1660–1661 (F7–MDGs, 1660–1661) from the VUL collection. It presents Part IV of the critical catalogue of the manuscripts written in Polish and Old Byelorussian (Ruthenian) languages, also as short descriptions of the documents No. 664–774 (pp. 709–826). These documents are important for the research of Lithuanian, Polish and Byelorussian history. Information accumulated within them reflects a broad political, social and cultural panorama of the multilingual GDL society. It enables us to consider them as unique reference books that represent the process of change in the sociolinguistic situation of the GDL.