A simple technique for measuring the thermal diffusivity of dielectric films on thermally sinking substrates is proposed and demonstrated. It is an outgrowth of the thermal pulse technique for measuring charge and polarization profiles. The thermal diffusivity is derived from the transient electrical response induced by a thermal pulse applied to a dc voltage‐biased sample. Because the response is proportional to the bias voltage, the signal‐to‐noise ratio is adjustable independently of the thermal pulse energy and may be made as large as required for determining the diffusivity with high precision. Measuring times of around 1 ms or shorter are sufficient for polymers with thicknesses up to 10 &mgr;m. Experimental results for two different polyimide films spin coated on crystalline silicon substrates are presented and discussed. ©1996 American Institute of Physics.