Object (clitic) pronouns in European Portuguese exhibit a degree of positional flexibility: although they normally occur postverbally, the presence of any one of a specific set of syntactic elements in pre-verbal position causes object pronouns to appear preverbally. Moreover the combination of preverbal pronoun and verb appears to exhibit a degree of syntactic transparency. We argue that while in both cases the pronoun and the verb are combined morphologically, the status of the morphological operations responsible for pre-verbal and post-verbal placement differ. On the basis of the EP data, we argue for a certain degree of complexity in the interface between the morphology and syntax. We then discuss the syntactic environments requiring pronominal prefixation, drawing parallels with data in other (non-Romance) languages and showing how such requirements may be captured in a constraint based formalism.