The effect of interfacial decohesion, due to mismatch of firing temperature, on the thermal diffusivity of a hot-pressed cordierite matrix with a dispersed phase of AlN was investigated by the laser flash technique at room temperature. The interfacial gap formed due to varying degree of sintering acts as a barrier to heat flow and lowers the thermal diffusivity. The resulting conductivity is compared with the Rayleigh-Maxwell and Bruggeman model for predicting the thermal conductivity of a composite. The study also investigates the processing parameter on the retention of AlN in the composite after hot pressing.