Ultra-high temperatures ceramics (UHTCs) are the subject of intense worldwide research effort, and their stability in severe environments makes them candidates for aerospace, nuclear and solar energy applications. Widespread usage UHTCs requires the development of effective and reliable joining methods that facilitate the fabrication of large, complexshaped, and potentially multimaterial components and devices. Joining of HfB2 and ZrB2, UHTC diborides, which exhibit outstanding thermo-mechanical and thermochemical properties and good erosion and corrosion resistance, was the focus of the present study. MoSi2 is an effective sintering aid and a composite component for both HfB2 and ZrB2, resulting in dense bulk materials with excellent mechanical properties. HfB2 –10 vol.% MoSi2 composites were joined at 1500 °C with a Ni/Nb/ Ni interlayer that forms a thin liquid fi lm. Joint-region characterization revealed well-bonded interfaces with interfacial reaction products with the MoSi2. Well-bonded interfaces were also obtained for a ZrB2 –10 vol.% MoSi2 composite bonded at 1500 °C with both Ti and Zr interlayers. It was found that the Ti interlayer exhibited more intensive interfacial reaction with ZrB2 composite than the Zr interlayer. Additionally, well-bonded interfaces were also found for a ZrB2 –10 vol.% MoSi2 composite bonded at 1500°C with ZrB2 -X vol.% Ni (X = 20, and 40) powder-based interlayer. Joint-region characterization revealed well-bonded interfaces with microstructures strongly dependent on the Ni content.