Sessile drops of liquid uranium on five refractory oxide substrates were studied photographically. The volume expansion on melting (ca. 3%), the contact angles (135 ± 3° in each case) and the surface tension of the liquid (826 ± 10% dynes/cm.) were determined. The reactions of liquid uranium with alumina or magnesia proceed stoichiometrically to the expected products. The reactions with zirconia, thoria, and beryllia yield oxygen-deficient substrates and only small amounts of the corresponding metal in solution. The kinetics and reaction mechanism are discussed. Some effects of alloying on these phenomena are also noted.