The reduction of the cobaltocenium cation (Cc+) to uncharged cobaltocene (Cc0) and to the cobaltocenium anion (Cc−) has been studied voltammetrically in acetonitrile in the presence and absence of supporting electrolyte using platinum microdisk electrodes of radii from 0.5 to 6 μm. In the presence of supporting electrolyte, the voltammetry is observed as two well-separated diffusion-controlled one-electron-reduction processes Cc+ + e− ⇆ Cc0 and Cc0 + e− ⇆ Cc−. However, precipitation of neutral cobaltocene formed as a result of the initial one-electron-reduction process and possibly adsorption of Cc+ appears to be induced by the removal of supporting electrolyte. The precipitation leads to rapid electrode passivation at the reversible Cc+/0 electrode potential and gives rise to the observation of a stochastic process at very small microelectrodes at more negative potentials. The voltammetry in the presence of electrolyte in higher dielectric solvent media such as water and dimethyl sulfoxide in which cobaltocene is known to be insoluble also exhibits features analogous to those observed in acetonitrile in the absence of electrolyte. The electrochemical behavior in the absence of supporting electrolyte is therefore consistent with the interfacial solvent having a higher dielectric constant than bulk solvent at negative potentials where reduction of the cobaltocenium cation takes place.