The voltammetry of water insoluble microcrystalline forms of [Co(mtas)(2)](ClO4)(3), [Co(mtas)(2)](ClO4)(2) and [Co(mtas)(2)](BPh(4))(2) (mtas = bis(2-(dimethylarsino)phenyl)methylarsine), mechanically attached to bard plane graphite electrodes has been studied in aqueous perchlorate electrolytes. For [Co(mtas)(2)](ClO4)(3) and [Co(mtas)(2)](ClO4)(2) the Co(III)/Co(II) and Co(II)/Co(I) processes are consistent with a model of electron transfer within a thin layer of electroactive material on or near to the surface of microcrystalline particles, with the bulk of the solid undergoing diffusion-controlled reactions at relatively slow rates. Interestingly, oxidation of BPh(4)(-) is necessary before a pronounced voltammetric response occurs for [Co(mtas)(2)](BPh(4))(2). Experimental data suggest that ion-pairing effects and the hydrophobicity of the solid control the electrochemical behavior in the solid-state studies. The visible spectra of Co(III) and Co(II) complexes in solution and in the solid state are consistent with similar electronic and molecular structures for each oxidation state in both phases. However, the potential of the Co(III)/Co(II) couple obtained with [Co(mtas)(2)](BF4)(3) dissolved in aqueous medium is approximately 170 mV more negative than for the same couple in the microcrystalline solids. The Co(II)/Co(I) couple occurs at a similar potential in both solid and solution phases. Voltammetry in the aqueous solution phase at a glassy carbon electrode results in the deposition of a self-limiting, continuous film of electrochemically insulating [Co(mtas)(2)]Cl or [Co(mtas)(2)](BF4) and highlights the importance of the microcrystalline morphology for the observation of well-defined voltammetry of nonconducting solid materials.