This paper explores the diachrony of the Portuguese contrastive connective ao passo que (‘whereas’). First, we describe its syntactic and semantic properties in present-day European Portuguese. With this contemporary analysis in mind, we explore the semantic and syntactic changes from the PP ao passo que (lit. ‘at the step/pace that’) into first a temporal connective of simultaneity (‘at the same time as’) and, ultimately, a contrastive expression. The evolution of expressions with temporal meanings into contrastive ones has been documented in many languages. In our paper, we show that another related meaning, that of the gradual development of events that are temporally simultaneous, may also evolve into a contrastive meaning. We also examine the role of the syntax and semantics of the noun passo in this process. Furthermore, we discuss the internal analyzability of the connective and provide evidence for the retention of some internal syntax, which has implications for current theories on the nature of complex categories.