In this paper we provide an account of the historical development of Polish and Russian sibilants. The arguments provided here are of theoretical interest because they show that (i) certain allophonic rules are driven by the need to keep contrasts perceptually distinct, (ii) (unconditioned) sound changes result from needs of perceptual distinctiveness, and (iii) perceptual distinctiveness can be extended to a class of consonants, i.e. the sibilants. The analysis is cast within Dispersion Theory, and we provide phonetic and typological data supporting the perceptual distinctivenss claims we make.