In this work we demonstrate how the reduction state of the Q-pool determines the distribution of electron flow over the two quinol-oxidising branches in Paracoccus denitrificans: one to quinol oxidase, the other via the cytochrome bc1 complex to the cytochrome c oxidases. The dependence of the electron-flow rate to oxygen on the fraction of quinol in the Q-pool was determined in membrane fractions and in intact cells of the wild-type strain, a bc1-negative mutant and a quinol oxidase-negative mutant. Membrane fractions of the bc1-negative mutant consumed oxygen at significant rates only at much higher extents of Q reduction than did the wild-type strain or the quinol oxidase-negative mutant. In the membrane fractions, dependence on the Q redox state was exceptionally strong corresponding to elasticity coefficients close to 2 or higher. In intact cells, the dependence was weaker. In uncoupled cells the dependence of the oxygen-consumption rates on the fractions of quinol in the Q-pool in the wild-type strain and in the two mutants came closer to that found for the membrane fractions. We also determined the dependence for membrane fractions of the wild-type in the absence and presence of antimycin A, an inhibitor of the bc1 complex. The dependence in the presence of antimycin A resembled that of the bc1-negative mutant. These results indicate that electron-flow distribution between the two quinol-oxidising branches in P. denitrificans is not only determined by regulated gene expression but also, and to a larger extent, by the reduction state of the Q-pool.