Redox properties of the photosynthetic gene repressor PpsR and the blue-light photoreceptor/antirepressor AppA from Rhodobacter sphaeroides have been characterized. Redox titrations of PpsR reveal the presence of a two-electron couple, with an Em value of −320 mV at pH 7.0, which is likely to arise from the reversible conversion of two cysteine thiols to a disulfide. This Em value is very much more negative than the Em = −180 mV value measured previously at pH 7.0 for the disulfide/dithiol couple in CrtJ, the homolog for PpsR in the closely related bacterium Rhodobacter capsulatus. AppA, a flavin-containing blue-light receptor that is also involved in the regulation of gene expression in R. sphaeroides, contains multiple cysteines in its C-terminal region, two of which function as a redox-active dithiol/disulfide couple with an Em value of −325 mV at pH 7.0 in the dark. Titrations of this dithiol/disulfide couple in illuminated samples of AppA indicate that the Em value of this disulfide/dithiol couple is −315 mV at pH 7.0, identical to the value obtained for AppA in the dark within the combined experimental uncertainties of the two measurements. The Em values of AppA and PpsR demonstrate that these proteins are thermodynamically capable of electron transfer for their activity as an anti-repressor/repressor in R. sphaeroides.