In most of the literature on English phonology, historically prefixed words such as contain, respect or submit are seen as having no morphological structure synchronically. However, such words were treated as complex in the early generative literature and are still analysed in that way in part of the literature. In this paper, we seek to reviewthe evidence for the claimthat suchwords are simplex words,which predicts that they should pattern with words with no internal structure in their phonological and morphological behaviours and in psycholinguistic experiments.We show that the evidence does not support that claim and shows that these words should be treated as morphologically complex units, although they differ from words with productive morphology. As these words tend to be partly or entirely opaque semantically, this raises the question of how such structures may be learned. We argue that the recurrence of forms is the main factor leading to their identification and lay out a possible order of acquisition of various morphological structures. Finally, we argue that theories of phonology may account for this by allowing the reference to morphological constituents whose semantics are impoverished.