Tli reduction on gramicidin-modified dioleoylphosphatidylcholine-coated mercury electrodes has been studied by the impedance method in different experimental conditions. On one hand, experiments at twelve gramicidin concentrations in 0.1 M KCI solutions have bean performed in order to clarify whether radial diffusion plays some role. On the other hand, KCl, KNO3, KClO4, MgCl2, and CaCl2 solutions have been used as supporting electrolyte to study the influence of ion nature on both the mass transfer and the electron transfer of the process. In all the cases the frequency dependence of the admittance data fits well to a Randles circuit and the Warburg coefficient, a, and the irreversibility coefficient, p ', can be obtained at every de potential. However, the potential dependence of a can only be explained if a mechanism is assumed, including two chemical steps, one preceding and one following the electron transfer (CEC mechanism) even at very high gramicidin concentrations. The impedance equations for reactions on partially blocked electrodes have been deduced, but the results do not conform completely to this model, so other explanations are proposed for the origin of the two chemical steps. Moreover, the standard rate constant, k(s), and the charge transfer coefficient, a, for the electron transfer were obtained from p '. These parameters are not sensitive to gramicidin concentration or to the nature of the supporting electrolyte. On the contrary, the mass transfer parameters are strongly dependent on these variables. The behavior can be understood at the light of the mechanisms for ion movement in biomembranes.