In the system of ZrO2-Al2O3, cubic ZrO2 solid solutions containing up to 40 mol % Al2O3 crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO2 solid solution powders containing 25 mol % Al2O3 have been sintered at 1000-1150 degrees C under 196 MPa using the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of approximate to 50 nm exhibited a very high fracture toughness of 23 MN m(-1.5) They have been characterized by X-ray diffraction and electron probe surface analyses.