In specimens of uranium oxides irradiated at high heat ratings for a few minutes or several days, central melting can be distinguished from columnar or equiaxed grain growth which takes place below the melting point. Evidence of melting has been observed in six different types of fuel: (1) UO^2.0 pellets formed by cold compacting and sintering; (2) UO^2.12 pellets; (3) UO^2.19 pellets; (4) UO^2.0 — 4 molecular per cent Y²O³ pellets; (5) cold swaged UO^2.0; and (6) loose compacted UO^2.0. The results show that the integrated thermal conductivity of the fuel up to its melting point is higher for the stoichiometric pellets than for any of the other types tested.