|Abstract:||Since the work of Zubritskaya (1995) and Matushansky (2002), it has been known that prefixes and prepositions instantiate a synchronically unified and unique class in Russian phonology. A less discussed point is that Russian prefixes and prepositions cannot be unified on the basis of their morphosyntactic characteristics. An existing analysis (Rubach 2000) addresses only the phonological facts, implicitly assuming that the two categories are identical morphosyntactically. To resolve the apparent contradiction between the phonological identity and the morphosyntactic nonidentity of prefixes and prepositions, I propose a Stratal OT (Kiparsky 2000) approach to the jer vocalization and palatalization patterns within the complex containing the prefix/preposition and its host. The account takes as its foundation the work of Blumenfeld (2003), and posits that prefixes and prepositions are processed at distinct strata (word and postlexical, respectively). The paper also provides tentative evidence, based on jer realization patterns, to support the claim that there is a phonological, not just morphosyntactic, reason to associate the composition of prefixes and prepositions with different strata.