[en] A fast-response cryogenic boil-off calorimeter was used to measure decay heat from the products of thermal fission of ²³⁹Pu, ²³³U, and ²³⁵U. Data are presented for cooling times between 20 and 10⁵ s following a 2 x 10⁴-s irradiation at constant thermal neutron flux. The experimental uncertainty (1 sigma) was 3 to 4% for ²³⁹Pu, approximately 5% for ²³³U, and approximately 2% for ²³⁵U. The average percent deviation of the experimental data points from summation calculations using the ENDF/B-IV data base was +9.7% for ²³⁹Pu, +4.2% for ²³³U, and +0.5% for ²³⁵U. The agreement between experiment and calculation is satisfactory except in the case of ²³⁹Pu, where the deviation exceeds the combined uncertainties in the experiment and the calculation. The difference between experiment and summation calculation for ²³⁹Pu is believed to be significant, but its source has not been identified. 4 figures, 12 tables