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The temperature dependence of the partial reactions leading to turn-over of the UQH2:cyt c2 oxidoreductase of Rhodobacter sphaeroides have been studied. The redox properties of the cytochrome components show a weak temperature dependence over the range 280–330 K, with coefficients of about 1 m V per degree; our results suggest that the other components show similar dependencies, so that no significant change in the gradient of standard free-energy between components occurs over this temperature range. The rates of the reactions of the high potential chain (the Rieske iron sulfur center, cytochromes c1 and c2, reaction center primary donor) show a weak temperature dependence, indicating an activation energy < 8 kJ per mole for electron transfer in this chain. The oxidation of ubiquinol at the Qz-site of the complex showed a strong temperature dependence, with an activation energy of about 32 kJ mole-1. The electron transfer from cytochrome b-566 to cytochrome b-561 was not rate determining at any temperature, and did not contribute to the energy barrier. The activation energy of 32 kJ mole-1 for quinol oxidation was the same for all states of the quinone pool (fully oxidized, partially reduced, or fully reduced before the flash). We suggest that the activation barrier is in the reaction by which ubiquinol at the catalytic site is oxidized to semiquinone. The most economical scheme for this reaction would have the semiquinone intermediate at the energy level indicated by the activation barrier. We discuss the plausibility of this simple model, and the values for rate constants, stability constant, the redox potentials of the intermediate couples, and the binding constant for the semiquinone, which are pertinent to the mechanism of the ubiquinol oxidizing site.