A new technique is proposed for measuring thermal diffusivities of materials at high temperature. A thin‐slab specimen is heated with a steady flux of energy on one or both faces. The flux to one face is then interrupted momentarily during which time radiative heat losses cause a negative temperature pulse to develop in the specimen and to propagate through it. The thermal diffusivity is determined from an analysis of the temperature excursion of the opposite face. A theoretical analysis of this heat‐transfer problem for which one‐dimensional heat flow and a general linearized form of surface heat loss are assumed is presented. Data analysis techniques are discussed, and the results are given for some measurements on AXM‐9Q graphite to 3250 K. A 250‐W CO2continuous‐wave laser was used as the heat source.