The combination of redox potentiometry with low temperature electron spin resonance (ESR) spectroscopy has led to further characterization of electron transfer components of Chromatium D. These include the readily buffer-soluble cytochromes c 553 and c′ and the high-potential iron-sulfur protein in the isolated state and associated with the chromatophore membrane. Buffer-insoluble cytochrome c 553, cytochro—me c 555, bacteriochlorophyll and the primary electron acceptor have been characterized both in the chromatophore membrane and also in a sodium dodecylsulfate detergent-solubilized subchromatophore preparation. Two iron-sulfur proteins have been revealed which are present in the chromatophore membrane but are released on treatment with sodium dodecylsulfate. They have central g values at 1.90 and 1.94 and have estimated midpoint potentials at pH 7.4 ( E m7·4) at +280 mV and −100 mV, respectively, when associated with the chromatophore. In the membrane associated state the apparent E m of cytochrome c′ is approximately 200 mV more positive than the E m values reported for the free state; this implies either that the reduced form of cytochrome c′ binds to the membrane (or to a component therein) to a degree which is > 10 3 times greater than that of the oxidized form or that the E m shift results from membrane solvation. In the case of the high-potential iron-sulfur protein however, its E m when associated with the chromatophore membrane is similar to that reported in the isolated state. The light-induced oxidation of the high-potential iron-sulfur protein at room temperature appears to be linked only to the oxidation of cytochrome c 555; it could serve as an electron pool in equilibrium with cytochrome c 555 in the cyclic electron flow system. The redox component defined in the reduced state by its g y = 1.82 and g x = 1.62 ESR spectrum satisfies the following criteria for its identification as the primary electron acceptor of P883. (a) The E m7·4 value of the g = 1.82 component is −120 ± 25mV. (b) At −70 mV, where the g = 1.82 component is mainly oxidized in the dark, brief illumination at low temperature which causes the irreversible oxidation of one cytochrome c 553 heme, also induces the permanent reduction of the g = 1.82 component; the extent of reduction after brief illumination, given by the g = 1.82 signal height, is the same as that induced chemically at −270 mV showing it to be fully reduced by the receipt of a single electron. (c) At more positive potentials where cytochrome c 553 is oxidized and is not involved in low-temperature reactions, the light-induced low-temperature kinetics of the g = 1.82 signal are reversible; the flash-induced g = 1.82 formation and subsequent dark decay are the same as those for the flash-induced P +883 ( g = 2) formation and dark decay. We suggest that until a full physical-chemical characterization is completed this g = 1.82 component be designated “photoredoxin”.