Abstract We have measured the thermodynamic efficiency of oxidative phosphorylation of isolated rat-liver mitochondria during oxidation of succinate. Furthermore, we have calculated what the effect of proton leak or slip in the redox pumps should be on the efficiency of energy transduction in oxidative phosphorylation. These calculations were compared with experiments in which the efficiency was determined in the presence of induced proton leak or redox slip. The results of these experiments are in agreement with the predictions. It is concluded that it is possible to distinguish experimentally between effects of proton leak and redox slip on energy transduction.