Six experiments examined why some idioms can be syntactically changed and still retain their figurative meanings (e.g., 'John laid down the law' can be passivized as 'The law was laid down by John'), while other idioms cannot be syntactically altered without losing their figurative meanings (e.g., 'John kicked the bucket' cannot be passivized into 'The bucket was kicked by John'). Our thesis was that the syntactic behavior of idioms is determined, to a large extent, but speakers' assumptions about the way in which parts of idioms contribute to their figurative interpretations as a whole. The results of our studies indicated that idioms whose individual semantic components contribute to their overall figurative meanings (e.g., 'go out on a limb') were judged as more syntactically flexible or productive than nondecomposable phrases (e.g., 'kick the bucket'). These findings suggested that idioms do not form a unique class of linguistic items (e.g., as "dead" metaphors), but can share many of the same compositional properties normally associated with more "literal" language. The implications of these data for theories of syntactic productivity of idioms and for models of idiom comprehension are discussed.