Cyclic voltammetry was used here to investigate the kinetics of the electron transfer between a soluble iron(III) complex and several cytochromes isolated from Desulfovibrio and Desulfuromonas species. The second order rate constants were calculated and found to be in the 10(5)-10(6) M-1 s(-1) range. Their values are discussed from the point of view of the redox potentials, heme content and pi value of the proteins. The ability of poly(ester-sulfonic acid) ionomer (Eastman AQ-29D) to entrap positively charged cytochromes made it possible to study the immobilized proteins. When the cytochromes are incorporated into this polymer matrix, diffusion limitations lead to a 50 times lower catalytic efficiency as compared to catalysis with protein in solution. The results of this study demonstrate that only cytochromes with bishistidinyl heme iron coordination act as metal reducers whereas mitochondrial c-type cytochromes do not. This approach opens new pathways for the use of sulfur and sulfate reducing bacteria in the bioremediation of metal contaminated waters and waste streams. Processes involving the use of entrapped enzymes reactors could be developed according to the metal reducing activity of their polyhemic c-type cytochromes.