The temperature dependence of the fracture toughness of ceramics exhibiting duplex microstructures was studied relative to their single‐phase constituents using two test methods: bend testing of chevron‐notched beams, and the indentation‐crack‐length technique. The two materials systems studied were Al2O3:c‐ZrO2(Y) and A12O3:Y3A1SO12(YAG), and the testing temperature ranged from room temperature to 1200°C. The study showed that in both systems the duplex materials showed higher toughness values than their single‐phase constituents above 800°C. This result was attributed to the contribution of low‐energy interphase boundaries to the overall composite toughness. Indentation crack length measurements gave toughness values and trends comparable to those determined by the chevronnotched beam method. By comparing the results of the two test methods it was possible to demonstrate that the indentation calibration constant (ξ) shows no significant temperature or material dependence. For the zirconia‐containing materials, however, indentation at elevated temperatures is accompanied by significant localized plasticity, which suppressed the radial cracking. Under such conditions, some caution is warranted, since localized plasticity can lead to an overestimation of the