The mobility of micron-sized powders of refractory and noble metals in UO2 was investigated under isothermal and temperature gradient conditions. The metal particles were initially placed between two polished surfaces of UO2, and any movement that occurred during high temperature annealing was determined microscopically. Tungsten and molybdenum particles 1 to 10 µm in diameter were immobile in UO2 at 2500°C in a temperature gradient of 1400°C/cm. Ruthenium, however, dissolved into and spread through hypostoichiometric, polycrystalline urania and was found after isothermal annealing as the U-Ru intermetallic compound in the grain boundaries of the oxide. The mechanism does not involve bodily motion of the metal particles. Rather, ruthenium dissolves in the grain boundaries of the oxide, migrates as atoms via the same pathway, and reacts while migrating to form URu3. This product grows as layers in the grain boundaries. Isothermal ruthenium spreading followed simple diffusion theory, and apparent solubilities and effective diffusivities were obtained from the data for the temperature range 2000 to 2300°C. In a temperature gradient, ruthenium moves to the hot zones of UO2; the mechanism appears to be the same as found for isothermal spreading, but the extent of movement up the temperature gradient cannot be explained by simple diffusion theory, even with an appreciable Soret effect.