This paper compares the relative merits of liquid-phase sintered β-Si 3N 4 with sintered α-SiC for high-temperature applications. These materials represent two extremes of ceramic microstructure: liquid-phase sintered β-Si 3N 4 contains grains that are coated by a second phase, whereas sintered α-SiC contains grains that are in direct crystalline contact. As will be shown, the mechanical behavior of the two materials differs substantially. At temperatures up to 1500 °C, sintered α-SiC is a creep-resistant solid. At room temperature, however, it is brittle, K Ic =(2–4) MPa·m 1/2, and has a low bending strength, σ b =(400–500) MPa. By contrast, liquid-phase sintered β-Si 3N 4 is not as creep resistant since it contains a residual sintering aid at its grain boundaries that deforms at a lower temperature than the silicon nitride grains. Hence, its temperature capability is less than that of sintered α-SiC. Silicon nitride is, however, tougher, K Ic =(6–8) MPa·m 1/2, and stronger, σ b =(700–1000) MPa, than sintered α-SiC. Deformation of liquid-phase sintered β-Si 3N 4, and other ceramics with a second phase at the grain boundaries, depends on the refractoriness of that phase, the more refractory the phase, the more resistant the material is to creep. Experimental results on β-Si 3N 4 suggest that toughness decreases as creep resistance increases; hence, a trade-off must be made between creep resistance and material toughness to achieve an optimal high temperature microstructure.