Screen-printed carbon electrodes (SPCEs), both plain and modified with a polycarbonate membrane, have been investigated as base transducers to study the electrochemistry of cytochrome c in phosphate buffer; cyclic voltammetry was used for all studies. It is proposed that the plain SPCE behaves as an array of microelectrodes, therefore mass transport occurs by radial diffusion and a sigmoidal response is obtained. When a polycarbonate membrane is attached to the SPCE the voltammogram is peak shaped indicating a change from radial to linear diffusion. It is suggested that the membrane acts as a 'solid state promoter', which increases the density of active sites for electron transfer to the heme centre of the metalloprotein. In this case, the distance between sites of electron transfer decreases with the result that diffusion layers overlap and a linear diffusion model prevails. Possible mechanisms of interaction of cytochrome c with surface active sites are postulated. (C) 2001 Elsevier Science Ltd. All rights reserved.