The microstructure of two pressureless-sintered ultra-high-temperature ceramics, namely ZrC+20 vol% MoSi2 and HfC+20 vol% MoSi2, was characterized by scanning and transmission electron microscopy. With regard to the ZrC-MoSi2 system, ZrxSiy compounds and SiC were detected. In the HfC-MoSi2 system, a mixed phase was detected at the triple points and identified as (Mo,Hf)(5)Si-3. For both the systems investigated, the high wettability of the silicide-based phases on the matrix grains suggests that sintering is assisted by a liquid phase. This contribution reports for the first time on the sintering mechanisms of early transition metal carbides doped with MoSi2 as a sinter additive, on the basis of the microstructural evolution observed upon sintering and in the light of phase diagrams and thermodynamical calculations.