A model is presented for the calculation of transient combined radiative/conductive heat transfer in heterogeneous semitransparent materials at elevated temperatures. It is based on optical material properties (scattering, absorption) that can be determined experimentally at room temperature. This model is applied to the simulation of laser-flash measurements on ceramic powder compacts. We demonstrate that the relevance of the radiative contributions for the determination of the thermal diffusivity from laser-flash measurements depends on the sample thickness, which is confirmed by corresponding experiments. It is shown that the thermal diffusivity of 1-2 mm thick alumina powder compacts may be overestimated by about 10-20% at 850 degrees C.