Phase relationships among solids in the UO2-O2-H2O system at 25, 100, and 200°C and pressures to 2 MPa have been calculated from critically evaluated thermodynamic data. Stability limits of the solids are expressed in terms of oxygen and water partial pressures at each temperature. The results are then discussed in terms of known UO2 oxidation reactions and uranium mineralogy. Particular attention is paid to “UO3 hydrates, ’’some of which are shown to be stable phases in air at very low relative humidities (down to ∼0.1% at 25°C). This is relevant to fuel storage because of the very high molar volumes of these phases, relative to UO2, and consequent potential for damage to defected fuel assemblies. Comparison of the calculated phase relationships with observed UO2 oxidation behavior helps to identify those phase interconversions that are kinetically constrained.