Abstract Chlorophyll a (Chl a) and cytochrome c were adsorbed on successive layers on a hanging mercury drop electrode and their interaction was studied in the dark and under illumination. The presence of Chl a between the electrode and the cytochrome prevents the protein from undergoing denaturation. The CO group of the cyclopentamone ring of the Chl a acts as a promoter of the cytochrome electron transfer. The presence of aggregated species of Chl a is revealed by the split of the reduction peaks of the molecule. The cytochrome c is photoreduced by the polymer of the Chl a, and in its reduced state, acts as a donor for the photo-oxidized bacteriochlorophyll a.