Uranyl peroxide or uranium peroxide hydrate (UO₄·nH₂O) is a pale-yellow, soluble peroxide of uranium. It is found to be present at one stage of the enriched uranium fuel cycle and in yellowcake prepared via the in situ leaching and resin ion exchange system. This compound, also expressed as: UO₃·(H₂O₂)·(H₂O), is very similar to uranium trioxide hydrate UO₃·nH₂O. The dissolution behaviour of both compounds are very sensitive to the hydration state (n can vary between 0 and 4). One main characteristic of uranium peroxide is that it consists of small needles with an average AMAD of about 1.1 µm.