Decyl aurachins C and D are two synthetic compounds related to the natural products aurachins C and D extracted from Stigmatella aurantiaca. Titrations of a range of partial respiratory activities in membranes from facultative phototrophs indicate that decyl aurachin C is more effective than aurachin D in inhibiting the quinol oxidase. Decyl aurachin C also affects the NADH-ubiquinone oxidoreductase of Rhodobacter capsulatus and the cytochrome bc1 complexes of Rb.capsulatus and Rhodospirillum centenum. Titration of the light-induced respiration in Rsp.centenum allowed us to estimate an upper limit for the ubiquinol oxidase concentration of 1.5 +/- 0.2 nmol/mg protein, or a ubiquinol oxidase/cytochrome c oxidase ratio of 3:1.