Fission-product decay energy-release rates have been measured for thermal-neutron fission of 235U. 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 for each set of time-interval parameters. The irradiations were for 1, 10, and 100 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 pulse of fissions, and the other following an infinite period of irradiation. A complete representation of estimated uncertainties is given in the form of a variance-covariance matrix. For the pulse representation of the data, diagonal components correspond to uncertainties in the range of 3 to 4%, with correlation coefficients in the range from 0.1 to 0.5. The experimental data are compared with other experimental data. The present results are generally smaller than other data, in some cases by more than the estimated uncertainties. The present results are smaller than the proposed 1973 American Nuclear Society (ANS) Decay-Heat Standard by as much as 10% for times following fission between 2 and 400 s, and are also smaller than the presently proposed (1978) ANS Decay-Heat Standard by 5 to 8% for the time interval 2 to 600 s. The reasons for these differences are discussed, and the importance for analyses using the new standard is presented.