Cytochrome c (cyt c) was immobilized on surface-modified gold electrodes using a self-assembling approach. The resulting cyt c electrode was studied using cyclic voltammetry. Compared to pure phosphate buffer, cyt c electrodes exhibited in DMSO-containing solutions lower oxidation and reduction peak currents, which originated from a decrease in the addressable electro-active amount of the surface-immobilized protein. This is associated with the process of protein denaturation. The denaturation kinetics can be described by a sum of two processes with time constants differing by more than one order of magnitude. The subsequent change of the aqueous/organic medium back to a pure aqueous buffer resulted in a shift of the formal potential to its initial value and a partial recovery of the peak current. This can be attributed to the renaturation of the cyt c. The extent of renaturation depended on the organic solvent/water ratio of the mixture used. The kinetics of protein renaturation were similar to those of the denaturation process. (C) 2004 Elsevier B.V. All rights reserved.