The microstructures and some mechanical properties of composites containing mullite and each of three different zirconias stabilized with low concentrations of yttria, magnesia, and ceria, have been studied. A sol-gel derived, high-purity, mullite was used as a matrix phase. In the present study, composites were prepared by conventional sintering of mullite and zirconia milled powder mixture. In all the composite materials, targe fractions of the tetragonal zirconia (ZrO2) transformed into monoclinic form during cooling from the fabrication temperature. In the use of ceria-stabilized ZrO2, large internal macroscopic voids appeared in the sintered body. The thermal expansion hysteresis associated with the tetragonal/monoclinic transformation was evident only in the mullite/yttria-stabilized ZrO2 composite from which the M-s temperature could be determined.