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The initial solid phase oxidation products formed during the oxidation of aqueous Mn (II) at 25° C were studied as a function of time. The analyses included morphology (TEM), mineralogy (x-ray diffraction), O Mn ratio (iodometric method), oxidation state of manganese (XPS), and dissolved manganese. The initial solid formed under our conditions was Mn 3 O 4 (hausmannite) which converted completely to γMnOOH (manganite) after eight months. βMnOOH (feitknechtite) appeared to be an intermediate in this transformation. The O Mn ratio was initially 1.37 and increased to 1.49 over the same time span. Throughout the course of this study the XPS analyses showed that the surface of the solids (< 50 Å) was dominated by Mn (III). The solution pH and dissolved manganese concentrations were consistent with disproportionation and oxidation reactions that favor the transformation of Mn 3 O 4 to γMnOOH but not to …