Most second language (L2) learners are aware of the importance of vocabulary, and this awareness usually directs their attention to learning new words. By contrast, learners do not often recognise unfamiliar idioms if all the compositional parts look familiar to them such as "turn the corner" or "carry the day." College-level ESL (English as a second language) learners (n = 52) at intermediate to advanced levels of proficiency participated in a series of metalinguistic tasks designed to examine their explicit recognition of unfamiliar idioms that may only look familiar because of their familiarity with the words in the idioms. Results show that L2 learners tend to recognise unfamiliar idioms with familiar components at a significantly lower rate than they recognise unfamiliar words such as "serendipitously" or "octogenarian." In addition, L2 learners' definitions of idioms indicated as familiar tend to be significantly lower in acceptability than their definitions of words indicated as familiar. It is argued that, if this pattern is repeated over time, the opportunities for learning these idioms incidentally might be reduced. It is suggested that learners need to become more aware of the common presence of these idioms in language, and that learners' continued awareness is key to sustained acquisition of these idioms.