[en] Summation calculations of fission product yields, half-lives, and decay energies are used to develop a standard of decay heat resulting from fission of U-235 and Pu-239 by thermal neutrons. Results are presented in tabular and graphical form. Uncertainties in input data and their cumulative effect on the uncertainty associated with the standard is evaluated. Also analyzed are the effects of neutron capture by fission products, fission of U-238 by fast neutrons, irradiation time, and depletion and conversion of fissile species. The new standard is compared to the proposed ANS-5.1 standard (1973) and to preliminary data on decay heat measured at several laboratories