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Calculations show that oxidation of chromium oxide (Cr₂O₃) by oxygen and oxidation of chromium hydroxide (Cr(OH)₃) by manganese dioxide (MnO₂) are thermodynamically feasible in both aerobic and mildly anoxic environments. Experiments were carried out to determine the rate and extent of chromium oxidation under various conditions, i.e., when Cr₂O₃ was heated in the presence of oxygen, when Cr(OH)₃ and MnO₂ mixtures were suspended in aerobic or anoxic aqueous media at various pH values, when Cr(OH)₃ and MnO₂ mixtures interacted in moist aerobic conditions and when chromium assumed to be Cr(OH)₃ and manganese assumed to be MnO₂ interacted in the presence of competing electron donors/acceptors, as is the case in chromium-contaminated sludge. Results indicate that trivalent chromium in Cr₂O₃ could be readily converted to hexavalent chromium at a temperature range of 200–300 …