The electrical effects occurring during freezing of dilute aqueous solutions have been studied using NH4OH and NH4Cl solutions. The freezing potentials depend on the concentration of the solution and the rate of growth of the solid phase. This effect is an impurity effect and does not appear in pure material. The phenomenon can be explained by a model in which the impurity rejected by the growing solid into the solution ionizes, one kind of ion being adsorbed on the solid surface at the interface and the other kind diffusing into the liquid. The impurity ion adsorbed on the surface moves with the surface and is not incorporated into the solid phase. This effect is confined to a narrow range of concentration of the solutions.