Ceria based solid solutions are important materials for their electrochemical applications. The synthesis and use of cerium oxide-based systems can take place at high temperatures. In this case, the selective evaporation of volatile components from the condensed phase occurs, which leads to a change in the composition and properties of the system. The data on high temperature stability of ceria-based ceramics is lacking in the literature. In the present work we report the thermodynamic stability investigation of CeO2-Gd2O3 solid solutions. Synthesis of the samples in the CeO2-Gd2O3 system was carried out by solid state synthesis method at the temperature of 1400 °C during 48 h. The characterization of the samples was performed by the X-ray diffraction method (XRD) and X-ray photoelectron spectroscopy (XPS). The vaporization of the CeO2-Gd2O3 system was studied by the Knudsen effusion mass spectrometry (KEMS) in the temperature range 1900–2300 K. Using the obtained data on component activities in the condensed phase, the thermodynamic properties of solid solutions in the CeO2-Gd2O3 system—such as Gibbs energy and excess Gibbs energy—were obtained as a function of ceria molar fraction. It was shown that the activity of ceria has a slight negative deviation from ideal case in all concentration range, whereas the activity of Gd2O3 changes from positive deviation at low concentration of ceria to negative deviation at high concentration.