The black layer was generated underneath the anode when a gold-electroded silver metaphosphate glass, a silver-ion conductor, was dc biased with 500 V/cm or 250 V/cm at 140degreesC . It was concluded that this layer corresponded to the silver-depleted region formed by conduction of the silver ions towards the cathode. The blackening was ascribed to be due to the shortened Ag-O distance, caused by structural rearrangement within the region into the P2O5-like structure on biasing.