A precision gamma-scanning system was applied to the nondestructive determination of the isotopic distributions of 103Ru and 106Ru in two failed (U,Pu)O2 fuel pins irradiated in the Experimental Breeder Reactor II. Significant differences in the two distributions were measured that were related to the half-lives of the two ruthenium isotopes. The two distributions can be used to determine the relative times at which the ruthenium deposits were formed within the fuel material. This information may be used to determine nondestructively the changes within the central void region over a period of several irradiation cycles.