The thermal decomposition of mixed Zr-Mn oxide hydrates containing 0-0.3 mol of manganese was examined by means of simultaneous TG and DTA. When heated, these mixed hydroxides decompose to an amorphous oxide phase and subsequently crystallize to a cubic phase. These changes are accompanied by a broad endothermic peak at around 165-180 degrees C and an exothermic peak between 450 and 700 degrees C. The temperature at which the crystallization occurs depends on the manganese content. A linear increase was observed between manganese content and crystallization (exothermic peak) temperature. The enthalpy of crystallization, however, showed a decreasing trend (from 28.7 to 12.1 kJ mol(-1)) with increasing manganese content within this range. Since no amorphous manganese oxide was formed when manganese oxide hydrate alone was heated, it is apparent that optimum concentrations of manganese (0.1-0.3 mol) in amorphous zirconia decrease the enthalpy of crystallization, thereby stabilizing zirconia in the cubic phase at low temperatures.