The potential of Al2O3/SiO2 additives for the retention of cesium in coated particles of high-temperature gas-cooled reactors is elucidated by fundamental studies of the system Cs2O−Al2O3−SiO2. Samples with nine different compositions were prepared by applying hydrothermal crystallization. Their phase composition was studied by x-ray diffraction extending the knowledge about the phase diagram. The vaporization of the samples was investigated with high-temperature mass spectrometry. From the partial pressures obtained for cesium, the efficiency of the various cesium aluminosilicate phases for the retention of cesium can be determined. The cesium pressures together with the phases observed in the samples provide the basis for cesium retention as a function of the optimum composition and quantity of the Al2O3/SiO2 additives. By comparing the cesium partial pressures over the various cesium aluminosilicates with those that are necessary for the formation of cesium lamellar compounds in reactor-grade graphite and pyrolytic graphite, it is found that lamellar compounds cannot be formed in coated particles if cesium aluminosilicates are formed.