Introduction: The chemical stability of bacteriogenic UO2 is the seminal issue governing its success as an “in-situ waste form” in the subsurface. The goals of this project are to identify key molecular-scale processes that mediate the stability and oxidation of bacteriogenic UO2 in the subsurface and to measure their rates at the meter scale. We will eventually develop coupled (bio)geochemical conceptual process models describing the effects of these processes on U(VI) release from UO2 in heterogeneous subsurface media. Hypotheses: We postulate that: (I) the stability of bacteriogenic UO2 is enhanced by cation substitution into UO2 ; (II) that sulfides may locally buffer the redox potential of ground water and protect UO2 from oxidation; and (III) that subsurface bacterial Mn(II) oxidation may catalytically enhance UO2 oxidation.