ENIn the Vilnius archives we still find new, unexplored (or only partially researched) materials. It seems that the great value of these materials lies primarily in the fact that the facts contained in them are of great importance not only for Polish studies, but also Lithuanian. The non-Slavic aspect is the one that I would like to emphasize in research on archival materials written in Polish and underline that not always Polish and Lithuanian studies have been conducted simultaneously. Such a tendency has only emerged in the last decade, and the need to look at similar phenomena from different research perspectives only broadens the horizons of knowledge and reveals new research issues. I used two examples in my considerations. The first concerns the activity of Jan Karłowicz in Lithuania and his contribution to Lithuanian linguistics, highlighted by Lithuanians. The second example is the multilingual legacy of the 17th-century Jesuit preacher and lexicographer Konstanty Szyrwid, analyzed from the Polish language perspective. Keywords: Vilnius archives, Polish and Lithuanian studies, Jan Karłowicz, Konstanty Szyrwid, multilingual legacy. [From the publication]