We investigated the extent to which the literal meanings of the words forming literally plausible idioms (e.g., "break the ice") are semantically composed and how the idiomatic meaning is integrated in the unfolding sentence representation. Participants read ambiguous idiom strings embedded in highly predictable, literal, and idiomatic contexts while their EEG was recorded. Control sentences only contained the idiom-final word whose cloze values were as high as in literal and idiomatic contexts. Event-related potentials data showed that differences in the amplitude of a frontal positivity (PNP) emerged at the beginning and at the end of the idiom strings, with the idiomatic context condition associated with more positive voltages. The time frequency analysis of the EEG showed an increase in power of the middle gamma frequency band only in the literal context condition. These findings suggest that sentence revision mechanisms, associated with the frontal PNP, are involved in idiom meaning integration, and that the literal semantic composition of the idiomatic constituents, associated with changes in gamma frequency, is not carried out after idiom recognition.