In situ rotating ring disk electrode (RRDE) and simultaneous electrochemical-electron spin resonance (SEESR) techniques have been employed to characterize the products and the phases of the products formed during voltammetric reduction or oxidation of microcrystalline solids mechanically attached to an electrode which has been placed in a solvent (electrolyte)medium in which the components are insoluble. Results obtained for reduction of solid organic 7,7,8,8-tetracyanoquinodimethane (TCNQ)and oxidation of solid organometallic trans-Cr(CO)(2) (dpe)(3) [dpe : 1,2-bis(diphenylphosphino)ethane)] show that the two compounds exhibit similar mechanistic pathways despite their substantially different chemical and physical natures. Data obtained from both RRDE and SEESR techniques show that a solid-state nucleation/growth mechanism confined to the electrode surface dominates the electrochemical processes at potentials where voltammetric peaks are observed. Thus. for example, in solvent (electrolyte) combinations where the product of electrolysis ([TCNQ](-) and trans-[Cr(CO)(2)(dpe)(2)](+)) are insoluble, characteristic solid-state electron spin resonance (ESR) spectra are obtained. In contrast, solution phase ESR spectra are obtained when the electrode is placed in media in which the products are soluble. Data obtained confirm that the conductivity associated with the solids does not play a significant role in the voltammetric response oi solid microcrystalline compounds when mechanically attached to an electrode surface.