The importance of the thermal diffusivity of restorative dental materials is well established. The laser flash method first described by Parker et al. (1961) is widely used to determine the diffusivity of industrial materials but has not hitherto been reported in the study of dental materials. The effect of the crystalline alumina content of dental porcelains has been investigated using this technique which permits rapid and accurate measurement. Values of 1.45 mm2/s and 0.66 mm2/s were determined for a commercial aluminous core porcelain and a dentine porcelain respectively. Mixtures of core and dentine powders exhibited intermediate levels. Increasing the alumina content resulted in increased diffusivity. The highest value was determined for a new high strength aluminous core porcelain which yielded a diffusivity of 1.74 mm2/s. This material was found to contain an increased concentration of alumina particles.