A series of pressurized water reactor spent-fuel samples from Turkey Point Unit 3 have been oxidized at temperatures between 140 and 225°C in air atmospheres with dew points between 14.5 and -70°C, using a thermogravimetric analysis system. Tests lasted between 400 and 2100 h. At the conclusion of a test, the atmosphere was sampled to determine the release of fission gas during testing, and the fuel samples were analyzed for microstructural changes. It appears that the mechanisms for oxidation of spent fuel to U3O7 takes place in two steps that occur somewhat simultaneously. Oxygen migrates along the grain boundaries, which are oxidized and enlarged. The grains oxidize by the inward progression of a layer of U4O9 saturated with oxygen. A simplified model of the mechanism, which considers oxygen diffusion through the product layer as the rate-controlling step, yields an activation energy of 113 ± 17 kJ/mol. Moisture, between dew points of —70 to +14.5°C, i.e., water vapor partial pressures varying over four orders of magnitude, had no significant effect on the oxidation rate.