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The oxidation-reduction potential of cytochrome b of heart muscle was estimated by Ball (1938) to be -40mv at pH 7-4 and 250. Since then it has become increasingly evident from the work of Chance (1952a, b) on the kinetics of the respiratory chain and from measurements by Hill (1954, and personal communication) that the true value of the potential is likely to be considerably more positive. In the work reported here we have reinvestigated the equilibrium between the bound cytochrome b of heart-muscle particles and the succinate-fumarate system, by using spectrophotometric methods which make small changes in the oxidation state of particle-bound pigments readily measurable (Holton, 1957) and employing a method of calculation which does not require experimental data relating to complete reduction of cytochrome b. Our results show that that portion of the cytochrome b which is functional in the succinicoxidase system of the heart-muscle particles (see Colpa-Boonstra, 1959) possesses an oxidationreduction potential which at pH 7 is some 120 mv more positive than the value estimated by Ball at pH 7-4. These results have been briefly reported elsewhere (Colpa-Boonstra & Holton, 1959). The experiments of this paper consist largely of measurements of changes of extinction which are attributed wholly or in part to one or other of the respiratory pigments present in the heart-muscle particles. Interpretation of the data depends upon a knowledge of details of the difference spectra of the individual pigments (reduced state minus oxidized state for the respiratory-chain pigments, deoxygenated state minus oxygenated state for myoglobin). For convenience, Fig. 1 summarizes such data compiled from various sources. They form the basis for much of the discussion which follows.

Spectrophotometric observations relating to the oxidation–reduction potential of cytochrome b in non-phosphorylating heart-muscle particles