Rationale: The TiO ₂–Al ₂O ₃–SiO ₂ system is the base for various glass–ceramic materials, which have great practical value for a large number of modern technologies. Many TiO ₂–Al ₂O ₃–SiO ₂ materials are synthesized or applied at high temperatures, which justifies the relevance of the present study. Methods: The samples in the TiO ₂–Al ₂O ₃–SiO ₂ system were synthesized using the method of induction melting in a cold crucible. The thermodynamic properties of the TiO ₂–Al ₂O ₃–SiO ₂ system were studied using the Knudsen effusion mass spectrometric method. The derived thermodynamic functions were optimized within the generalized lattice theory of associated solutions (GLTAS) approach and compared with the results of calculation using the semiempirical Kohler, Muggianu, Toop, Redlich–Kister, and Wilson methods based on the corresponding data in the binary systems. Results: The SiO ₂ selective vaporization from the samples under study was shown at temperatures above 1940 K. The thermodynamic properties in the TiO ₂–Al ₂O ₃–SiO ₂ system, including the TiO ₂–SiO ₂ system, were obtained in the temperature range 1965–2012 K and were optimized using GLTAS to obtain the consistent concentration dependences of the component activities and excess Gibbs energies. Conclusions: Positive deviations from the ideal behavior were observed in the TiO ₂–Al ₂O ₃–SiO ₂ system at high temperatures. Comparison of these values with the results of the modeling based on the GLTAS approach allowed the recommendations regarding the optimal semiempirical methods for the excess Gibbs energy calculation in different concentration ranges to be made.