The kinetics of several electron transfer reactions involving cobalt cytochrome c (Cocyt c)1 and iron cytochrome c have been determined. The results are as follows: oxidation of Cocyt c by Fecyt c+, k = 8.3 X 103 M"1 s_1 (µ = 0.04-0.2 M), AH* = 2.3 kcal mol-1, AS* = -33 eu; self-exchange reaction of Cocyt c,k < 133 M"1 s_1 (µ = 0.1 M); autooxidation of Cocyt c, k = 12.3 AF1 s'1 (µ = 0.001-0.1 M); mediated reduction of methemoglobin by Cocyt c,k = 5.2 X 104 AT1 s-1 (µ = 0.49 M), AH* = 6.3 kcal mol™1, AS* = -18 eu for the case of phenazine methosulfate; and the oxidation of Cocyt c by Fe(EDTA)~, k = 68 AT1 s~x (µ = 0.1 M), AH* = 4.2 kcal mol"1, AS* = -36 eu. The rate constants are those for 25 °C, pH 7.0. The results were analyzed in terms of the nonadiabatic multiphonon electron tunneling theories of Hopfield and of Jortner. Also included in the analysis are the following electron transfer processes: self-exchange of Fecyt c, forward and reverse electron transfer between Fecyt c and Pseudomonas cyt c551, reduction of Fecyt c+ by Fe(EDTA)2-, oxidations of Fecyt c by Co(phen)33+ and by Fe(CN)63+, autoxidations of Fecyt c and FeHb, reversible redox reactions of Fecyt c2 and of Fecyt c with Fe(CN)6, reduction of Fecyt c+ and Cocyt c+ by S2042-and by S02~•, oxidation of bovine cardiac cytochrome cl by Fe(CN)63" and its reversible electron transfer with Fecyt c, and the reactions of reduced cytochrome oxidase with oxygen. Using the known midpoint potentials and approximate distances of closest approach, and two experimentally determined values for the vibronic coupling parameters, the rate constants of the above reactions were calculated to agree better than a factor of 2 for most of the reactions and satisfactory agreement for the others. The theoretical values of AH* and AS* are also in good agreement with experimental results. This is quite remarkable in view of the diverse nature of the reactants and mechanistic pathways and the fact that their rate constants span eight orders of magnitude. It seems that analysis of electron transfer data should always include one using the nonadiabatic multiphonon electron tunneling theory. The theory has the potential of providing a unified framework for the interpretation of electron transport between biological molecules.