Peculiarities of phase formation in the ternary oxide system Cs2O—Al2O3—SiO2 have been studied using samples prepared from precursors obtained by hydrothermal synthesis followed by calcination of the reaction mixtures at different temperatures for different times. The samples were identified by high-resolution X-ray powder diffraction (HRXPD), simultaneous thermal analysis, and X-ray fluorescence spectroscopy. A HRXPD study revealed that all the samples contained an X-ray amorphous phase and crystalline phases. According to HRXPD data, the crystalline phases in the samples with the compositions Cs2O : Al2O3 : SiO2 = 33.0 : 33.0 : 33.0 and 30.0 : 50.0 : 20.0 (mol.%) corresponded to cesium aluminosilicate CsAlSiO4, while those in the sample containing equimolar amounts of the oxides corresponded to CsAlO2, CsAlSiO4, and Cs2CO3·3H2O formed on the surface of the sample. An X-ray fluorescence spectroscopy study demonstrated that the elemental composition of the heat-treated samples remains close to the intended stoichiometric composition.