Abstract By means of flash spectrophotometry, the oxidation reactions of cytochromes c 2 and b can be differentiated. Cytochrome c 2 oxidation had a risetime of 1–2 msec; that of cytochrome b was several times longer. Both risetimes were several orders of magnitude longer than the duration of the excitation flash, suggesting that both cytochromes were oxidized in the dark. Unique for the cytochrome- b transients was the appearance of sharp spikes occurring always in the direction opposite of those of the cytochrome- b oxidation. These transient spikes occurred at an excitation intensity level beyond the saturation level for both cytochromes c 2 and b. The risetime for the sharp spike transients was approx. 50 μsec. In the wild-type Rhodospirillum rubrum cells, reactions due to the carotenoids caused spectral changes in the 470–560-mμ region. This resulted in an unproportionately large negative α- and β-bands in the light- minus-dark difference spectrum. There was no noticeable difference in the decay kinetics between the cytochromes and the carotenoids. The oxidation of both cytochromes c 2 and b was stopped near o°. At −196°, positive bands appeared at 430 and 790 mμ, and negative bands appeared at 380, 810 and 870 mμ. All reactions taking place at the liquid-nitrogen temperature had a risetime of approx. 50 μsec. The low-temperature difference spectrum probably represents changes originated from the bacteriochlorophyll molecules. The net effect of 4 inhibitors ( p-chloromercuribenzoate (PCMB), phenylmercuriacetate, 2- n-heptyl-4-hydroxyquinoline- N-oxide (HOQNO), and antimycin A) on R. rubrum cells was similar, i.e., the light- minus-dark difference spectrum resembled closely that obtained with whole cells at −196° or from chromatophores at room temperature. The effect of PCMB on the transients at all wavelengths was immediate. In the presence of phenylmercuriacetate, HOQNO or antimycin A, however, the final transient profile at 420 mμ took more than 2 h to become established. Prior to that time, the transient underwent a series of intermediate stages. More unusally, the 420-mμ transient in the presence of HOQNO or antFSimycin A appeared to completely revert to the initial profile at approx. 120 min after the inhibitor was added.