[en] Calculations of reactor emergency cooling requirements are of considerable importance in the design of power reactor systems. The reliability of such calculations can be assured only by comparison with precise measurements of the fission product decay heat. The present work was designed to provide the required measurements for the two fissile materials of primary concern in current thermal reactors, ²³⁵U and ²³⁹Pu. In an attempt to provide a wide range of experimental data in order to facilitate the interpretation of possible discrepancies between measurements and calculations, data were taken for three different irradiation times, 1,000 and 20,000 seconds, and 24 hours. These different irradiation times place emphasis on different fission product nuclides. In addition, the beta and gamma-ray components of the decay heat are resolved based on an auxiliary measurement employing a beta absorber around the irradiated sample during the measurement. The cooling time spanned by the present work, 1 to 100,000 seconds, encompasses the time interval of prime concern for loss-of-coolant accidents