Actinide oxo clusters are an important class of compounds due to their impact on actinide migration in the environment. The photolytic reduction of uranyl(VI) has potential application in catalysis and spent nuclear fuel reprocessing, but the intermediate species involved in this reduction have not yet been elucidated. Here we show that the photolysis of partially hydrated uranyl(VI) in anaerobic conditions leads to the reduction of uranyl(VI), and to the incorporation of the resulting U-V species into the stable mixedvalent star-shaped (UI)-I-V/U-V oxo cluster [U(UO2)(5)(mu(3)-O)(5)-(PhCOO)(5)(Py)(7)] (1). This cluster is only the second example of a (UI)-I-V/U-V cluster and the first one associating uranyl groups to a non-uranyl(V) center. The U-V center in 1 is stable, while the reaction of uranyl(V) iodide with potassium benzoate leads to immediate disproportionation and formation of the (U12U4VO24)-U-IV cluster {K(Py)(2)K(Py)[U16O24(PhCOO)(24)(Py)(2)]} (5).