The studies on the history of phraseology usually examine idioms that either expired or have survived until nowadays. We rarely hear about changes of meaning of idioms. Phraseological units, as stable combinations of words, are more resistant to change of meaning than individual lexemes. In the article the authoress investigates the idioms that underwent different kinds of semantic transformations in the course of time. Changes of meaning are the starting point to show the causes that determine those changes. They occur most frequently as a result of the obliteration of the primary motivation of an idiom (demotivation). The changes of the extralinguistic reality are closely related to this aspect. With the same frequency, competing phraseological synonyms and variantability of phraseological units decide about the change of semantic values. The significant cause of changes of meaning lies also in the modification of the formal structure of idioms, including proverbs. To some extent, changes of meaning assure the stability of phraseological units.