The article deals with general consistent patterns and variation conditions of the meaning of English phraseological units of verbal and substantive types in context. As the result, some potential and maximal possibilities of semantic variation of verbal and substantive phraseologisms - idioms of the modern English language were revealed, and some conditions of contextual variation of phraseological units in usual and occasional use have been discovered as well. The research dwells on comparison of zones of constant and variable semantic features of English phraseological units. As the result, some interrelation between semantic variation of phraseologisms in context and theme - rheme meaningful decomposition of utterance and thematic blocks in English dialogical speech have been disclosed.