Reduction of uranyl(VI) to U-V and to U-IV is important in uranium environmental migration and remediation processes. The anaerobic reduction of a uranyl U-VI complex supported by a picolinate ligand in both organic and aqueous media is presented. The [(UO2)-O-VI(dpaea)] complex is readily converted into the cis-boroxide U-IV species via diborane-mediated reductive functionalization in organic media. Remarkably, in aqueous media the uranyl(VI) complex is rapidly converted, by Na2S2O4, a reductant relevant for chemical remediation processes, into the stable uranyl(V) analogue, which is then slowly reduced to yield a water-insoluble trinuclear U-IV oxo-hydroxo cluster. This report provides the first example of direct conversion of a uranyl(VI) compound into a well-defined molecular U-IV species in aqueous conditions.