ZrB 2 - and HfB 2 -based composites containing 1, 3, and 9vol.% MoSi 2 as sintering aid were densified by spark plasma sintering at temperatures ranging from 1700 to 1950°C, with an applied pressure of 100MPa. As received ZrB 2 and HfB 2 powders were also sintered at temperatures in the range of 2100–2200°C. Fully dense samples were obtained between 1750 and 1850°C when MoSi 2 was added. As received ZrB 2 powder was sintered to 98% at 2100°C. The microstructural features were investigated by means of SEM–EDS technique. Silicon carbide was detected in all the doped compositions along with significant amounts of oxide species (Hf/ZrO 2 and SiO 2 ). Vickers hardness and fracture toughness were measured by indentation technique on polished sections. The effect of the MoSi 2 content on densification, microstructure and mechanical properties is discussed.