Measurements have been made of the effective thermal diffusivity at room temperature of composites consisting of one phase randomly dispersed in a second phase. The method is based on the flash technique. Data are presented for four types of composites ranging in particle-to-matrix diffusivity ratios from 0.48 to 1137, in volume specific heat ratios 0.04 to 1.16, and in volume fraction of dispersed particle from zero up to 34 percent. The results show that the limitations of the concept of an effective thermal diffusivity are far beyond the situations to which it is currently applied in the transient state heat conduction problems. Values of effective diffusivities derived from values of the effective thermal conductivity calculated from the Bruggeman variable-dispersion equation are found to agree well with the measured diffusivity values.