The redox properties of cytochromes c are remarkably sensitive to pH, ionic strength, ionic composition of the medium and temperature. The effect of increasing concentrations of a variety of anions on the redox potential of mitochondrial and bacterial species measured through voltammetric techniques indicate the presence of specific sites of interaction on the protein surface. Proton uptake and release and specific anion binding alter the protein charge and their effect on E° allow information to be obtained on electrostatic effects on the redox properties of the heme. The temperature dependence of the redox potential of cytochrome c changes with pH. At alkaline pH values a T-induced structural change occurs above 40 °C.