The results of an experimental study of the behavior of fission products released from metallic uranium and a U-3.5%Mo alloy into helium and air are reported. A thermochromatographic technique for investigating the nature of low concentration gas species is described and applied to the release of barium, lanthanum, cerium, and molybdenum from irradiated fuels. Barium, lanthanum, and cerium, which deposit at high temperature in helium, are not significantly released into air because of the low volatility of their respective oxides. Molybdenum, which is quite nonvolatile in helium, is released as MoO3 in air.