This paper deals with a class of morphological alternations that seem to involve syntactic adjacency. More specifically, it deals with alternative realizations of syntactic terminals that occur when a particular phrase immediately follows a particular head. We argue that this type of allomorphy is not conditioned by a syntactic adjacency condition. Instead, it is found when the head and phrase in question are contained in the same prosodic phrase at the interface that connects syntax and phonology (PF). We illustrate our approach with six case studies, concerning agreement weakening in Dutch and Arabic, pronoun weakening in Middle Dutch and Celtic, and pro-drop in Old French and Arabic.