AbstractIn binary or quasi‐binary systems the superposition of chemical diffusion and field‐driven migration may lead to unstable diffusion fronts. This means that an initially one‐dimensional transport geometry may become three (two)‐dimensional. This concept is tested with the solid solution AgCl‐NaCl aboveT=200°C, which is above the critical point of the miscibility gap. Morphological instabilities of the concentration profile in the inter‐diffusion zone are found if an external electric field is applied. This field must exceed a critical value and the AgCl‐side must be anodic in order to induce an instability of the diffusion front. Numerical solutions of the transport equations and the given boundary conditions corroborate the experi