The detection of a xenon tag released from an element with breached cladding has proved to be the best method of identifying a source of fission product release in the Experimental Breeder Reactor II. From experience gained from December 1974 to September 1977 in identifying the sources of 25 major releases, allowance can now be made for the composition changes in xenon tags brought about by irradiation and for the presence of more than one tag in the cover gas. This experience has also shown that calculated ratios of fission gas to tag gas in subassemblies can be used in identifying a source of release. An ancillary method, which uses the measured 131Xe/ 134Xe ratio in the cover gas, can also determine whether or not the source is uranium. The success of these recent improvements is reflected in the short time to identify a source and in the small number of other sources—frequently none—that are suspect.