The article examines several groups of non-trivial vocabulary in the Book of Isaiah in the Septuagint: transliterations (Hebrew names written in Greek characters, religious terms and words obscure to the translator as well as Aramaic loanwords); neologisms (words appearing only in the Septuagint and related texts, primarily semantic calques based on morphemic translation from Hebrew) and semantic neologisms (words used with meanings different from their most frequent use in classical and Hellenistic texts and dependent on their Jewish equivalents); vocabulary typical of papyruses and inscriptions; literary vocabulary and rhetorical techniques characteristic of “high” classical literature. With reference to each group, the author summarises the issues most discussed in the academic literature on the book, namely basic approaches to describing the Greek language of the Septuagint; the issue of how the Greek Pentateuch’s influence on the lexical choice of the translator on the Book of Isaiah; the reflection of the colloquial norms of Alexandrian Jews and the proximity of the Septuagint language to the language of documentary sources of the same period; possible Aramaic influence; discussions about who the translator was.