The problems that might result from the release of fission gases in mobile fuel fast reactors are considered for two types of mobile fuel systems; namely, a molten alloy fuel system of the type to be used in the Los Alamos Molten Plutonium Reactor Experiment and a paste fuel system of the type being developed by the Atomic Power Development Associates, Inc. It is shown that the volume of fission gases generated in fast reactors operating at high-power density would supersaturate such fuel systems in minutes or less. An examination of the physical conditions in the reactor core and an evaluation of the phenomena responsible for bubble formation result in the conclusions that neither fuel system will sustain a significant degree of supersaturation and that bubble formation will most likely occur at a solid-liquid interface rather than in the bulk of the liquid. The effects of bubble formation in each system are considered, and these are seen to involve partial blanketing of the heat transfer surfaces, overheating of the fuel—particularly of the paste fuel, equilibrium dilution of the fuel with significant loss in reactivity, sudden displacement of the fuel with subsequent rapid changes in reactivity, and blocking of narrow fuel ligaments and orifices. Preliminary experiments, using supersaturated solutions of carbon dioxide in water and in water-glass bead beds are reported, which verify some of the analyses which are made regarding the location of bubble formation and the growth of bubbles. The flow characteristics of pastes in tubes and the behavior of gas bubbles in such flow systems are discussed in the light of experiments which were conducted using a simulant system of air/glass beads/water.