The thermal decay times of thin indium films evaporated on sapphire and quartz single-crystal substrates have been measured over a range of temperatures which, with earlier measurements, now extends from 4°-300°K. At the higher temperatures the results were found to be in good qualitative agreement with the predictions of the thermal diffusion theory of heat transport, although there were some quantitative discrepancies. The decay times characterizing the initial part of the thermal decay of a 2200-Å film on a sapphire substrate were measured to be as fast as ∼2×10-9 sec near 100°K, increasing to about 8×10-9 sec at room temperature. The decay times were found to be longest (up to ∼30×10-9 sec) below 10°K, where the heat transport is predominantly by ballistic phonon flow. The decay times for a 2700-Å film on a quartz substrate were found to lie between 13 and 30×10-9 sec in the range of temperatures from 4°to 300°K. © 1966 The American Institute of Physics.