A secondary phase has been observed to nucleate on the surface of UO2 in a solution with uranium concentration values of similar to 10(-9) mol/1. The UO2 was in the form of a 100 nm single crystalline film of UO2 epitaxially deposited on the (0 0 1) surface of a single crystalline silicon substrate. An extended (140 days) dissolution experiment with UO(2 )in( )contact with a solution in deoxygenated, deionised water, under an Ar atmosphere (0.1 O-2 ppm) at ambient temperature (similar to 25 degrees 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.