The microhardness (H) of single crystal, orthorhombic mullite was measured on (010) and (001) faces from room temperature up to 1400degreesC. The microhardness versus temperature curves display sigmoidal shapes. The mean microhardness at room temperature is similar to16 Gpa and it decreases with temperature to H is similar to 13 GPa at 300degreesC. At >300degreesC, the microhardness is only slightly reduced with temperature to H is similar to10 GPa at 1000degreesC. Above this temperature limit, it markedly decreases again, to a mean value of approximate to6 GPa at 1400degreesC. Up to similar to1000degreesC the (010) and (001) microhardnesses are very similar. Above similar to1000degreesC, however, the hardness gradually becomes anisotropic, with H-(010) being twice as high as H-(001) at 1400degreesC (H-(010) similar to 8 GPa, H-(001) similar to 4 GPa).