This paper discusses uranium oxide crystal structure modifications that are observed during the low-temperature oxidation which transforms UO2 into U3O8. The symmetries and the structural parameters of UO2, beta-U4O9, beta-U3O7, and U3O8 were determined by refining neutron diffraction patterns on pure single-phase samples. Neutron diffraction patterns were also collected during the in situ oxidation of powder samples at 483 K. The lattice parameters and relative ratios of the four pure phases were measured during the progression of the isothermal oxidation. The transformation of UO2 into U3O8 involves a complex modification of the oxygen sublattice and the onset of complex superstructures for U4O9 and U3O7, associated with regular stacks of complex defects known as cuboctahedra, which consist of 13 oxygen atoms. The kinetics of the oxidation process are discussed on the basis of the results of the structural analysis.