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The oxidation of Cr (III) by a common naturally oc-curring form of manganese oxide,-02, was charac-terized over a range of Cr (III) concentrations and pH values. Cr (III) oxidation was limited as pH and Cr (III) concentrations increased. Reaction products, Mn (II) and Cr (VI), did not limit Cr (III) oxidation. Initial Cr (III) oxidation rates were very rapid at pH= 5, but were sub-sequently followed by a dramatic rate decline. Thermodynamic calculations using solution species indicated the reaction should proceed under conditions where the re-action had terminated. A surface alteration induced by Cr (III) at pH> 3.5 appears to prohibit the extent of ox-idation. Various mechanismsmay account for the elec-trophoretic mobility and oxidation reactions; however, surface precipitation of chromium hydroxide seems to be the most plausible explanation based on our results. Chromium oxidation was dependent on Cr (III) concen …