Gas tagging consists of the addition to nuclear reactor fuel pins of small amounts of gas having a unique isotopic composition for each assembly; when an assembly fails during subsequent irradiation, the tag gas, which is released along with the fission gas, makes it possible to locate the defective assembly by a mass spectrometric analysis of the reactor cover gas. The usual gas tagging scheme employs only xenon; calculations are presented here which have led to the synergistic use of xenon and krypton for the fast flux test facility (FFTF) reactor. The ratios of the tag gas isotopic concentrations have been obtained for a preliminary design for the FFTF.