The retention behaviour of fission products in the material is therefore of special importance. 1 MeV iodine and cesium ions were implanted into yttria-stabilized zirconia (YSZ) and their depth profile was measured by Rutherford Backscattering after subsequent thermal treatments. While iodine becomes significantly mobile only above approximately 1500 K, cesium already starts to move at a temperature between 1000 and 1200 K. In a sample implanted with both species significant stabilisation of cesium is observed. In this case, cesium atoms stay immobile up to 1500 K.