Absolute fission-product decay energy-release rates have been measured for thermal-neutron fission of 239Pu and 241Pu. Spectral data were obtained using scintillation spectrometers for beta and gamma rays separately and were processed to the form of total yield and total energy-release integrals per fission for each set of time-interval parameters. The irradiations were for 1, 5, and 50 (241Pu) or 100 (239Pu) s, and measurements were made covering times following irradiation from 1.7 to 13 950 s. The separate beta- and gamma-ray energy-release data were summed to obtain the total (β + γ) energy-release rates for the cases studied. The data are processed to provide two standard representations of decay energy release, the one following a fission pulse and the other following an infinite fission period. Complete representations of estimated uncertainties are given in the form of variance-covariance matrices for the first time. For the pulse representation of the data, diagonal components correspond to uncertainties in the range of 3 to 6%, with correlation coefficients in the range from 0.1 to 0.5. Comparisons with other experimental data show that the present results are generally smaller than the other data, in some cases by more than the estimated uncertainties. The present results are also smaller than those included in the current American National Standard Decay Heat Power in Light Water Reactors, ANSI/IANS-5.1-1979, for 239Pu by 2 to 4% for the time interval 2 to 14 000 s. For 241Pu decay heat, the present data are larger than previously obtained experimental 235U data but smaller than the adopted 235U standard in ANSI/ANS-5.1-1979. The importance of these comparisons for analyses using the new Standard is presented.