Coupled electron and proton transfer observed in flow-flash experiments on GO-inhibited mixed-valence cytochrome c oxidase is discussed in terms of a model proposed by Brzezinski and co-workers [J. Bioenerg. Biomembr, 1998, 30, 99-107], The model includes two redox states of the heme a/heme a(3) pair and two states, protonated and deprotonated, of a redox-linked group L, which is in contact with bulk solution via a proton conducting channel. The proton channel is represented by another protolytic group L ', which is in equilibrium with bulk solution, but not with group L. The theory reproduces the experimentally observed pH dependence of the slow kinetics of heme a reduction following dissociation of the enzyme-CO complex, and additionally predicts a pH dependence of the fast kinetics due to varying proton equilibrium between group L and bulk solution prior to dissociation. The rates of internal proton transfer between L and L ' in the reduced and oxidized states, and the bimolecular rate of protonation of L ' by bulk protons have been evaluated from the present theory and experimental data. The protonation rate of the group L in the reduced state of heme a(3) is k(on)(red) = 10(4) s(-1) From the observed pH dependence of the rate constant for the slow kinetic phase of backward electron transfer the rate of L ' protonation is estimated to be kappa ' (on) = 5 x 10(11) M-1 s(-1).