Ultra-high-temperature ceramic (UHTC) materials with a density of 94.5 ± 1.3% have been manufactured by hot pressing a (HfB₂–30 vol % SiC)–5 vol % Y₃Al₅O₁₂ composite powder, which has been prepared sol–gel method, at a moderate temperature of 1850°C (holding time, 30 min; pressure, 30 MPa). The oxidation resistance of this ceramic has been studied at elevated temperatures under the effect of a supersonic dissociated air flow (on a high-frequency induction plasmatron). The maximum surface temperature has been ~2550°C. An analysis of the kinetics of temperature changes depending on the heat load indicate that, when five percent of Y₃Al₅O₁₂ by volume of HfB₂–30 vol % SiC is introduced into the ceramics, the thermal conductivity of the material decreases. This is not critical from the point of view of the stability of the obtained sample to single temperature drops of ~700–1400°C in a few seconds with sharp heating and cooling of the sample. A decrease of ~35–40% in weight loss of the (HfB₂–30 vol % SiC)–5 vol % Y₃Al₅O₁₂) sample has been noted as the result of exposure as compared with unmodified analogs. Due to the presence of Y₃Al₅O₁₂ in the initial ceramics, there is also a certain amount of the stabilized phase of cubic HfO₂ in the oxidized layer in addition to the monoclinic.