The conditions required for the accurate measurement of the thermal diffusivity of solids at high temperature 800–1800 K by the Angströ m method with cylindrical geometry are studied. It is shown that the main error in the measurement arises from the difficulty in measuring the temperature accurately with contact thermocouples. On the basis of new experimental results on Cecorite and polycrystalline alumina, the effect of the finite size of the sensors and the consequence of the imperfect thermal contact between the sample and the sensors are investigated. Two models which allow a greater insight into the simultaneous influences of both effects are developed.