The electrochemical behavior of carbon paste electrodes (CPEs) was studied by chronoamperometry and cyclic voltammetry in solutions containing either potassium ferricyanide or ferrocenemonocarboxylic acid. An evaluation of the experimental results leads to the conclusion that the electron transfer reaction at CPE for ferricyanide is more consistent with the "pinhole model", whereas for ferrocenemonocarboxylic acid’ it may take place through the uniform phase ("membrane") of the pasting liquid. The electrochemical behavior of ferricyanide at glucose-oxidase-containing CPEs is discussed in terms of the electrostatic interaction of the negatively charged electroactive species with the positively charged protein domain.