The oxidation of Cr-III to Cr-VI in aqueous suspensions of Na-buserite (Na4Mn14O27. 9H(2)O) has been investigated as a function of the solution pH and solids concentration. The rate of Cr-VI formation in solution exhibits two apparent domains of rate control. At low solids concentrations the rate of reaction appears to be controlled by the transfer of an electron from Cr-III to the solid. At high solids concentrations the rate of Cr-VI formation appears to be controlled by the octahedral to tetrahedral coordination change that must occur during the oxidation of Cr-III to Cr-VI. A weak dependence of the rate of oxidation on the solution pH is observed, which is interpreted as evidence for an outer-sphere electron transfer process between Cr-III and the solid. Na-buserite pre-equilibrated at low pH (H-buserite) reacts considerably more slowly with Cr-III. This observation is correlated with an apparent retardation of Mn-II release from the H-buserite solid and suggests some equilibration of the aqueous chromium species with the solid phase. An energy level model of electron transfer is presented which allows some quantitative understanding of both the pH dependence of this reaction and the reasons for the reduced reactivity of the H-buserite phase.