Abstract: Novel transformation toughened ceramic compositions comprising finely divided ZrO.sub.2 -HfO.sub.2 solid solution particles dispersed in a Al.sub.2 O.sub.3 -Cr.sub.2 O.sub.3 (alumina) or 3Al.sub.2 O.sub.3.2SiO.sub.2 -3Cr.sub.2 O.sub.3.2SiO.sub.2 (mullite) solid solution matrix phase. It is found that increased hardness and modulus associated with higher relative Cr.sub.2 O.sub.3 content (e.g. 20 mole %) in the matrix phase and increased fracture toughness associated with higher relative HfO.sub.2 content (e.g. 10 to 20 mole %) in the disperse phase represent viable and improved ceramic compositions. Such transformation toughened ceramic compositions are considered useful in superhot light diesel engine applications, high temperature gas turbines and ultrahigh temperature ceramic heat exchangers.