Modern high-temperature technologies and methods of production of advanced materials impose new requirements on the quality of information on physicochemical properties of oxide systems at high temperatures. Normally, thermodynamic approach for these purposes is the most fundamental and essential. Great attention was paid by M.M. Shultz to extensive development of this approach in the studies of oxide melts, crystals, glasses, ceramics, and coatings using calorimetric, EMF, and high temperature mass spectrometric methods. Advantages of the thermodynamic approach were illustrated by examples of application of the Knudsen effusion mass spectrometric method to studies of oxide systems and materials, which were crucial for the further development of space and aviation industry, energetics, instrument making, communication engineering, metallurgy, energy-saving, and environmental safety. In the discussion of the regularities of vaporization processes and changes of thermodynamic properties in oxide systems, a number of particular systems based on silica and hafnia was considered in detail. Modeling was carried out for these systems using the Generalized Lattice Theory of Associated Solutions. The obtained results assert a necessity for creation of the national thermodynamic data- and model bases essential for further prediction of phase equilibria in oxide systems and materials at high temperatures.