Zirconium- and tantalum-based diborides, and diboride/carbide composites are of interest for ultra-high temperature applications requiring improved thermomechanical and thermochemical stability. This thesis focuses on the synthesis, processing and sintering of nanocrystalline powders with Zr- and Ta-based diboride/carbide/silicide compositions. A solution-based processing method was developed to prepare reactive mixtures that were precursors for ZrB₂-based powders. The precursors reacted to form the ceramic powders after suitable pyrolysis and borothermal/carbothermal reduction heat treatments. Single-phase ZrB₂ powders were prepared with initial composition of C/Zr = 4.8 and B/Zr = 3.0. ZrB₂-based composite powders with ZrC, ZrO₂, TaB₂, TaC, SiC, TaSi₂ and B₄C were prepared with particle sizes of 10-500 nm for different phases based SEM micrographs. The composite powders were highly sinterable with proper processing methods developed to avoid and remove oxide impurities. The relative densities of ZrB₂/B₄C, ZrB₂/TaB₂, ZrB₂/TaB₂/B4C, ZrB₂/TaSi₂ were in the range of 91%-97% after pressureless sintering at 2020 ℃ for 1 h or 30 min.