Abstract: The structure, mechanical characteristics, and high-temperature stability in vacuum and air of ZrB₂ and HfB₂-based materials sintered at a high quasi-hydrostatic pressure (4.1 GPa) under hot pressing (at a pressure of 30 MPa) with and without SiC and Si₃N₄ additives have been studied. It has been shown that short-term sintering (4 min) under high pressure conditions and at a comparatively low temperature (1800°C) essentially improves the mechanical properties of these materials as compared to the similar materials synthesized by the other method (hot pressing and spark-plasma sintering). In the case of sintering at a high pressure (4.1 GPa), the addition of 20 wt % SiC to ZrB₂ and 30 wt % SiC to HfB₂ leads to a decrease in the specific gravity of ZrB₂ and HfB₂ and increases their hardness by 17 and 46% and fracture toughness by 40 and 21%, respectively. When SiC is added, there occurs the formation of solid solutions through the mutual diffusion of C and Si into the ZrB₂ or HfB₂ matrix phases and the slight diffusion of Zr and Hf into SiC-enriched areas. The improvement of the mechanical properties of ZrB₂ and HfB₂ sintered at a high pressure without additives is explained by the formation of stronger bonds between the sintered material grains. The addition of SiC to ZrB₂ slightly decreases the Young modulus, but increases the damping ability of the synthesized materials. The simultaneous addition of SiC and Si₃N₄ to ZrB₂ leads to an increase in the hardness to a smaller extent, but results in a further increase in fracture toughness. The melting temperature in vacuum of sintered ZrB₂ and HfB₂ has proven to be much higher as compared to the materials with SiC additives. The composite material synthesized from a HfB₂–30 wt % SiC mixture has a density ρ = 6.21 g/cm³, a microhardness H_V(9.8 N) = 38.1 ± 1.4 GPa, H_V(49 N) = 27.7 ± 0.24 GPa, H_V(98 N) = 26.3 ± 2.03 GPa, and a fracture toughness K _Іс(9.8 N) = 8.2 ± 0.2 MPa m^0.5, K _Іс(49 N) = 6.8 ± 0.6 MPa m^0.5, K _Іс(98 N) = 6.4 ± 0.11 MPa m^0.5, which are much higher than the similar characteristics of HfB₂ sintered under the same conditions, but without the additives.