An ultra-high temperature ceramic based on ZrB 2 , TiB 2 , and SiC was hot pressed to full density at 1850 °C. Addition of 5 vol% of different metal-compounds, in the form of HfC, VC, NbC or CrB 2 , increased the densification temperature to 1910 °C. The resulting compositionally complex ceramics had homogeneous microstructures with boride grains exhibiting core-shell features. The shell was a solid solution containing variable amounts of the three metals. Notably, TiB 2 remained as a discrete phase in the reference material and in the presence of the V- and Cr- based additions, whilst it dissolved into the main ZrB 2 -based grains for other additives. Thermodynamic simulations and atomic size factors were exploited to explain the different solubility in the various systems. The compositionally complex diborides exhibited excellent properties at room temperature, with hardness up to 25 GPa and strength up to 800 MPa, which was preserved up to 1500 °C. However, increasing the testing temperature to 1800 °C resulted in plastic deformation owing to residual carbide phases. Electrical resistivity ranged between ∼13 and 70 µΩ·cm, with higher values in those ceramics where TiB 2 remained as a discrete phase. The observed overall properties improvements in compositionally complex borides pave the way for tailored design of UHTC materials with multication non-equiatomic composition for applications in extreme environments.