The oxidation of water-insoluble microcrystalline decamethylferrocene attached mechanically to a basal plane pyrolytic graphite electrode and then placed in aqueous media has been studied voltammetrically under a wide range of conditions. When dissolved in dichloromethane, the solution-phase voltammetry corresponds to an ideal chemically reversible one electron oxidation process : Fe(eta5 - C5(CH3)5)2 half arrow right over half arrow left [Fe(eta5 - C5(CH3)5)2]+ + e-. In contrast, the voltammetry of the solid is far more complex and exhibits a wide range of electrolyte, surface coverage, solvent composition and temperature dependences. Electron probe X-ray measurements show that anions are added to the surface of the solid when Fe(eta5 - C5(CH3)5)2 is oxidized and expelled on reduction of [Fe(eta5 - C5(CH3)5)2]+ so that chemically reversible ion transport across the solvent (electrolyte) boundary provides the mechanism to achieve charge neutralization. The potentials of the reduction and oxidation component of cyclic voltammograms vary considerably with the hydrophobicity of the electrolyte anion, but not the cation, and are also solvent dependent. If the electrolyte contains two anions, it is possible to observe two voltammetric processes. Increasing the scan rate or changing the surface coverage may lead to peak splitting so that both "thin layer" and "thick layer" type voltammetric responses are observed under some conditions. It is concluded that the mechanism for oxidation of microcrystalline decamethylferrocene mechanically attached to an electrode consists of electron transfer from the electrode into the solidsolution interface via the self-exchange mechanism coupled with ion transport across the solutionsolid boundary.