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Using AC impedance spectroscopy as a means of in situ thermometry, temperature runaway over 1,500°C within a few seconds was observed for the first time in dense 92 mol% ZrO2–8 mol% Y2O3 (8YSZ) ceramics heated under a constant voltage. Such runaway is especially pronounced when starting from a low background temperature in samples of a small surface-to-volume ratio, and is expected to be commonplace during flash sintering of porous ceramic cubes or cylinders. The method of AC impedance-based thermometry is applicable to other ceramics with a well-defined conduction mechanism at high temperature.