The article gives a concise history of investigation in modern linguistics of the problem of transformation of one part of speech into another one. Evaluated are some disputable statements. Attention is concentrated on the interaction of words in the system of unchangeable parts of speech — adverbs, prepositions, conjunctions, particles. The illustrative material is given to demonstrate the phenomenon of interaction and transformation that cause semantic changes in words and influence not only their structural and grammatical peculiarities but also their orthography in modern Ukrainian. The questions discussed in the article are closely connected with the syllabus and could be used while teaching the contemporary Ukrainian language at schools and higher educational establishments.