The chemical stability of bacteriogenic “UO2” is one of the seminal issues governing its success as an in-situ waste form in remediated subsurface locations. Little detail is known about the structure and reactivity of this material, but based on comparison to its closest abiotic analog, UO2+x (0 < x < 0.25), we expect that it is complex and defected, likely to exhibit non- stoichiometry, and capable of structurally incorporating common groundwater cations as well as U(VI). This complex behavior is expected to substantially impact its stability in groundwater. Our four-institution team is conducting a systematic and coordinated characterization of: (1) the atomic- and nano-scale structures of bacteriogenic UO2+x in the absence and presence of potentially important environmental cation dopants, (2) the equilibrium solubilities and dissolution rates of these materials, (3) the biogeochemical coupling of biologically mediated Mn cycling and UO2+x oxidation, and (4) the influence of these molecular scale processes on meter scale release of U(VI) in sediments. This presentation describes results from Year 1 of this project.