This paper provides an explanation for a sound change affecting Polish by which palatalized palatoalveolars became retroflexes. An extension of the account to a similar (but probably independent) Russian sound change is also considered. We argue that the sound change was motivated by the needs of perceptual distinctiveness within a rich sibilant inventory and provide an analysis within the framework of Dispersion Theory. This analysis is further supported by a typological survey and by phonetic data. This case study supports the view that "unconditioned" sound changes, and allophonic rules resulting from them, can be motivated by contrast, and further shows that the notion of dispersion in phonology can be usefully applied to consonants.