A secondary phase has been observed to nucleate on the surface of UO2 in a solution with uranium concentration values of ~10-9 mol/l. The UO2 was in the form of a 100 nm single crystalline film of UO2 epitaxially deposited on the (001) surface of a single crystalline silicon substrate. An extended (140 days) dissolution experiment with UO2 in contact with a solution in deoxygenated, deionised water, under an Ar atmosphere (~0.1 O2 ppm) at ambient temperature (~25 °C) suggests that uranium dioxide should dissolve and precipitate while remaining in the U4+ oxidation state to enable nucleation of a low solubility secondary phase. A mechanism for the anoxic dissolution of UO2 in deionised water is proposed that involves U4+ dissolution at defect sites that subsequently nucleate and precipitate in a less defective form.