Ion selectivity of nickel hexacyanoferrate [KNiFe(CN)₆] was determined by measuring cyclic voltammogram in sodium and cesium solution. The film electrode initially in the sodium form converted readily to the cesium form but the affinity of the electroactive film for cesium was still observed at the reverse processing in a concentrated sodium solution. The NHCF was found to be useful to the removal of radioactive cesium from the nuclear wastewater.