The article describes Church Slavonic loanwords' functioning and use in the lexical-semantic system of the Ukrainian written literary language of the XVI – first half of the XVII century. It discloses the concepts of «Church Slavonic language», «Church Slavonic loanwords». The composition and semantics of Slavonic loanwords in the language of various genres of Ukrainian memos (certificates, court documents, wills, diplomas, descriptions of castles, rounders Hetman of offices, documents of the Church and school fraternities, annals, works of religious polemic and imaginative literature, memos of scientific and educational literature, liturgical books etc.), walked to the source base of «Dictionary of the Ukrainian language of 16 – the first half 17 centuries» and its unique lexical card index are clarified. The study highlights specific features of the investigated borrowings in the Ukrainian language, specified time, set them apart from other words (Latinisms, Grecisms, Polonisms, and the like). These features are most active at the phonetic, word formation, and semantic levels of language. It is established that in the course of the historical development of the Church Slavonic language, some Church Slavonic loanwords, under the influence of the phonetic features of the Ukrainian language (the pronunciation of ancient etymological ы and и as a single sound in the front row and high raise [и]; use e in place of the stressed or unstressed ѣ; pronunciation of ѣ as i «new» ѣ at the place of the etymological e; writing e in place of Proto-Slavic ę; the alternation у/в; sound transformation in the individual prefixes and suffix; the softening of the ц; the transition in unstressed position у – о; loss of primary і; simplification in groups of consonants, etc.), changes in the sound structure of the word. Part of the Church Slavonic words, adapting to the semantic system of the Ukrainian language of 16 – the first half 17 centuries, changed to lexico-semantic level, particularly the expansion or narrowing semantics. Other old Church Slavonic lexemes are components of the diverse lexico-grammatical patterns of collocations and idioms connected with the Church ritual sphere. Keywords: adaptation, written memos, semantics, phrases, phonetic changes, Church Slavonicism.