The aim of the paper is to explore the process of r obstruentisation which can be found in Polish in the ‘trapped’ context, i.e. between two obstruents or at the end of the word after an obstruent. More specifi cally, in order to explain the mechanics of the [r] > [/] shift, the author looks at the phenomenon of sonorant devoicing and some historical facts concerning the development of r. Since r obstruentisation can also be found in a closely related Czech, the data from both languages are confronted and discussed. The solution offered here is based on the analysis of w obstruentisation proposed by Cyran and Nisson (1998).