Retention of cesium in yttria stabilized zirconia (YSZ) was studied experimentally and discussed on the basis of its solubility estimated on a thermodynamic basis. The retention was investigated experimentally by determination of the temperature dependence of diffusion and the cesium release. Cesium was introduced using 1 MeV ions or by chemical reaction in yttria stabilized zirconia. The distribution of the implanted cesium was evaluated by using TRIM computer calculation and measured by Rutherford backscattering spectroscopy (RBS) using 5 MeV 4He ions. The cesium profile was quantified after successive annealing up to 1373 K. Cesium release was rather weak up to 1173 K. The retention of Cs chemically introduced by Cs2ZrO3 was also experimentally investigated for different calcination temperatures. The chemically introduced cesium followed similar release behavior as the implanted one. Thermodynamic modelling of the system cesium, zirconium and oxygen was performed in a comprehensive way and as a function of temperature.