The thermal diffusivity of several natural and synthetic rubber compounds was measured in the temperature range from 125 to 350 K with an automatic device. In every case, a step change has been found in the thermal diffusivity at the glass transition range. In partially crystalline elastomers, the thermal diffusivity shows a minimum, which agrees very well with the melting temperature of crystallites. Also, new estimations of thermal diffusivity of some elastomeric compounds are given at above and below the glass transition temperature.