[en] Metatorbernite is a host of uranium that has been identified in the shallow vadose zone of the 300 A area at the Hanford site, WA, USA. Consequently, modeling the evolution of U concentrations in vadose zone porewaters driven by meteoric water recharge requires accurate knowledge of metatorbernite solubility. Previous determinations of the solubility constant for metatorbernite were under constrained. In the present contribution, the dissolution of natural metatorbernite crystals was studied at target pH 2.5 and 3.0, using both nitric and phosphoric acid. Steady state was approached from under- and super-saturation. The experiments and calculations yielded a preferred log K_sp = -28.0 ± 0.1 that is significantly different than previously determined values. Further, both stoichiometric and non-stoichiometric dissolution was observed as a function of pH and aqueous phosphate concentration.