The grammaticalization path reflexive > middle > anticausative > passive (> impersonal) has long been recognized as a well-attested pattern in grammar evolution (Kemmer, The middle voice, John Benjamins, 1993, Cennamo, The reanalysis of reflexives: A diachronic perspective, Liguori, 1993, Wehr, SE-Diathese im Italienischen, Günter Narr, 1995, Parry, Transactions of the Philological Society 96: 63–116, 1998, among many others). Yet, the history of individual constructions of reflexive origin in various Romance languages reveals a number of unexpected facts that call the directionality of this path into question. The article traces the history of Italian passive constructions in which a reflexive marker is used (the so-called si -constructions), and shows that the range of uses of si -constructions has contracted considerably from the first vernacular documents to present-day Italian. In particular, reflexive passives in which the agent is coded overtly were much more frequent in Old Italian than in later stages of the language, and have eventually disappeared in present-day Italian. While grammaticalization models are, strictly speaking, unable to account for this contraction (or, perhaps, are not concerned with it), the dynamics of the process are straightforwardly accounted for by diachronic models in which the notions of prototype and prototype effects play a role. The historical data discussed in the article reflect a process of polarization consisting in the functionalization of an (embryonic) formal and semantic contrast between the si -construction and another passive construction, the so-called periphrastic passive (formed with essere , ‘be’ + past participle). These two constructions are preferentially associated with two distinct constellations of semantic traits in present-day Italian, but not (or at least not so sharply) in Old Italian.