A method is proposed for determining the thermal diffusivity &agr; of a thin solid plate, mounted in a vacuum and heated to incandescence by means of a high‐energy electron beam impinging on one face of the plate, or heated by thermal radiation from an arc‐imaging furnace. The beam energy is to be modulated by either a square wave or a sine wave, and the resulting temperature modulation of the faces is to be observed photoelectrically. A theoretical study is made of the possibility of deducing the thermal diffusivity of the solid from amplitude and/or phase measurements. The most practical method seems to be that which involves sine wave modulation, and measurement of the phase difference between the temperatures of the two faces of the plate. With a plate thickness of about 1 mm and frequencies of the order of 0.01–300 cps (depending on the value of &agr;), it should be possible to measure thermal diffusivities, especially of the poorer conductors (&agr; less than about 0.1 cm2/sec), for temperatures from 1000°K or less to the point where sublimation becomes troublesome.