Two zirconium diboride-base composites were produced and characterised. The chosen starting compositions were: 55 wt.% ZrB₂+41 wt.%TiB₂+4 wt.% Ni and 83 wt.% ZrB₂+13 wt.% B₄C+4 wt.% Ni. The microstructure and properties of these composites were compared to those of a monolithic ZrB₂+4 wt.% Ni material. In all cases, metallic Ni as the sintering aid promoted the formation of the liquid phase which improved mass transfer mechanisms during sintering. From the powder mixture ZrB₂+TiB₂, two solid solutions of Zr–Ti–B were obtained. In the case of the other mixture, B₄C particles were dispersed in the ZrB₂ matrix. The composite materials have better mechanical properties than those of the monolithic ZrB₂ ceramic; in particular the fracture toughness and the flexural strength were almost doubled at room temperature. Long term oxidation tests indicated that the ZrB₂-based composites, particularly the composite containing B₄C as the second phase, were more resistant to oxidation than the monolithic ZrB₂ due to the formation of surface oxide products which were protective against the complete degradation by oxidation observed for the ZrB₂ matrix material.