The thermal diffusivity of a binary blend of poly(phenylene oxide) and polystyrene was investigated by the AC Joule heating technique in a practical temperature range including glassy state and melt state. The temperature dependence of the thermal diffusivity and the glass transition temperature (T-g) of the blends were clearly observed. The single T-g, characteristic for this blend, increased with increasing the blend ratio of poly(phenylene oxide). The thermal diffusivity was also dependent on the blend composition.