A very slow change in the dc conductivity below 200°C of certain alkali halide single crystals, not previously investigated, cannot be understood in terms of current theory of homogeneous crystals, but follows first‐order kinetics and correlates with dislocation substructure. Electrical decoration of the dislocation substructure suggests that conduction flow is inhomogeneous, a conclusion confirmed by experiments on bicrystals and on specimens containing subboundaries.