The article is based on an old prints language analysis of Medicines for dormant male intent by Demyan Nalyvayko (Ostrih, 1607), Mirrors of Theology by Kyrylo Stavrovetsky (Pochaiv, 1618), Eucharist by Sofroniy Pochasky (Kyiv, 1637). Shown is how important the colloquial Polish component was for an old-Ukrainian scribe, whose aim was to write his works “in an understandable manner”. It is focused on the fact that, despite the significant percentage of spoken Ukrainian elements in the texts of educated Ruthenians of the day, efforts s to create a colloquial text were linguistically made not only by employing the locally spoken Ukrainian. Numerous glosses, lexical doublets, syntactic constructions indicate the noticeable presence of Polish as a language in order to present the material to the reader in an understandable form. In the works of D. Nalyvayko, K. Stavrovetsky, S. Pochasky and many others, educated Ruthenians tended to write in a vernacular language embodied by the formula: local spoken Ukrainian plus Polish. There are many examples of the inclusion of structural elements from one language within the other, as shown by the analyzed material.