The study of high temperature behavior of Y ₃Al ₅O ₁₂ is urgently required for development of high temperature materials of the new generation including ultra-high temperature ceramics. The superfine Y ₃Al ₅O ₁₂ powder was obtained by the glycol-citrate method; the aggregation process of the sample synthesized was investigated under calcination conditions at different temperatures. Using the Knudsen effusion mass spectrometric method vaporization processes and thermodynamic properties of Y ₃Al ₅O ₁₂ as well as of the melts in the Y ₂O ₃-Al ₂O ₃ system were studied at temperatures higher than 2270 K. The simultaneous transition to the gaseous phase of both Al ₂O ₃ and Y ₂O ₃ from the melt under study was observed during isothermal keeping of the Y ₃Al ₅O ₁₂ sample at the temperature 2675 K with the predominant alumina removal. Analysis of the values of the thermodynamic properties obtained in the Y ₂O ₃-Al ₂O ₃ system showed negative deviations from ideal behavior despite the possibility of existence of the miscibility gap at the temperature 2675 K.