In the Liquid Metal Fuel Reactor (LMFR) under development in the United States, the fuel is a dilute solution of U, Mg, and Zr in bismuth. At the operating fuel temperatures (400–550°C), the volatile fission products (FPV's), which represent about ¼ of the total by weight, are mostly the noble gases Kr and Xe with small amounts of the halogen fission products Br and I. Owing to the facts that the LMFR is a thermal breeder reactor and that the 9.13-h Xe135 isotope has a 2.7 × 106-barn thermal cross section, the concentration of FPV's in the fuel and in the core must be kept very low for good neutron economy. For a 1 % reactor poisoning level, and assuming no Xe adsorbed on or absorbed in the graphite, the concentrations of 9.13-h Xe135 and total Xe in the fuel are estimated to be about 1.5 and 13 parts per billion, respectively, for a typical commercial plant. Complete isotopic compositions of the volatile fission products and poison levels for different removal rates are presented. The effect of various degrees of volatilization of the iodine and bromine on these factors are also shown. Xe represents over 80% by weight of the FPV's. Both Xe and its precursor, iodine, have strong tendencies to adsorb on unwetted surfaces and to penetrate graphite, the moderator material in the reactor core. Immobilization of Xe in the core would present a problem from the standpoint of reactor poisoning. Experimental results are presented to show the extents to which both iodine and Xe adsorb on steel and graphite and penetrate graphite. It appears that the Xe problem is not so much one of removing it from the fuel in a desorber as it is in preventing it from collecting on graphite surfaces in the core.