This document specifies a method for the determination of the thermal diffusivity of a thin solid disc of plastics in the thickness direction by the light flash method. This method is based upon the measurement of the temperature rise at the rear face of the thin-disc specimen produced by a short energy pulse on the front face. The method is applicable to homogeneous solid plastics as well as composites having an isotropic or orthotropic structure. In general, it covers materials having a thermal diffusivity, ±, in the range 1 × 10"7 m2Å"s"1 < ± < 1 × 10"4 m2Å"s"1. Measurements can be carried out in gaseous and vacuum environments over a temperature range from "100 °C to +400 °C. NOTE For inhomogeneous specimens, the measured values can be specimen thickness dependent.