In my thesis, I investigated the idiom passivization process from a cognitive linguistic point of view. According to this theory, only those idioms can be passivized whose literal and figurative meanings can be mapped onto one another, i.e. they are isomorphic. This isomorphism is guaranteed by various cognitive mechanisms like conceptual metaphors or metonymies. This hypothesis was tested in my two-part experiment in which I investigated 100 Hungarian and 100 English idioms. As in present-day Hungarian there is no sentence-level passivization, I examined idioms in the lett/lesz+V+-vA predicative adverbial participle construction. This structure resembles the passive voice to some extent; therefore, my assumption was that the cognitive model of idiom passivization should work with them. In the case of both languages, I used empirical data: the acceptability of the selected Hungarian expressions in the lett/lesz+V+-vA structure was measured by a grammaticality judgement test, whereas the passive form of the English idioms were checked in corpora. The results of my experiment supported the cognitive approach to idiom passivization in both languages. However, it should also be noted that there were some idioms that did not meet the expectations, and such instances suggest that there are some factors that can overrule this cognitive hypothesis about idiom passivization.