The thermochemical behavior of low-temperature oxidation in fine UO2 powders has been investigated by simultaneous thermogravimetric analysis and differential scanning calorimetry. Evaluation of the thermochemical and kinetic data reveals a complex interplay between different mechanisms. The initial reaction concerns the rapid chemisorption of oxygen gas onto the surface of UO2 grains, having an activation energy of only 13.1 ± 0.6 kJ mol−1. Subsequent oxidation at temperatures between 40 °C and 100 °C occurs first at the surface via a field-assisted mechanism, which progresses via domain growth into the bulk. At more elevated temperatures thermally activated diffusion becomes the dominant mechanism.