This paper reports a study of the high-temperature stability of ion-implanted yttria-stabilized zirconia and magnesium aluminate spinel, which are foreseen as matrices for the transmutation of nuclear waste. Stable analogs of radiotoxic fission products (Cs) were implanted into both materials and the stability of the implanted systems was studied upon annealing at a high temperature. Rutherford backscattering spectrometry experiments with a macro- and a micro-ion-beam, scanning electron microscopy, and atomic force microscopy observations show that the annealing behavior strongly depends on the material. In zirconia Cs begins to desorb out of the crystal at 550 °C in samples which remain homogeneous. On the contrary, the desorption of Cs is observed at 850 °C in spinel by the exfoliation of the surface of the samples