AbstractThe morphological evolution of initially planar phase boundaries between ionic crystals due to ion fluxes which are driven by an externally applied electric field is investigated. In this particular study, the boundaries under investigation are interfaces between KC1 and AgCl. It is shown that the moving boundary is morphologically unstable if the flux of silver ions is directed towards the KCl‐crystal, whereas it is stable if the flux of potassium ions is driven towards AgCl. This behaviour can be explained by the rules which derive from a formal stability analysis. The geometrical evolution of the moving phase boundary in its later stages is mainly governed by the elastic properties of the contiguous phase