The oxygen release kinetics of mixed-conducting Sr2Fe1.5Mo0.5O6 d–Sm0.2Ce0.8O2 d (SFM–SDC) dualphase composites has been investigated, at 750 C, as a function of the SDC phase volume fraction using electrical conductivity relaxation (ECR) under reducing atmospheres, extending our previous work on the oxygen incorporation kinetics of these composites under oxidizing conditions. Gas mixtures of H2/H2O and CO/CO2 were used to control step changes in the oxygen partial pressure (pO2) in the range 10 24 to 10 20 atm. At the conditions of the experiments, oxygen re-equilibration is entirely controlled by the surface exchange kinetics. A model is developed which allows deconvolution of the effective time constant of the relaxation process in terms of the intrinsic contributions of the components to oxygen surface exchange and synergetic contributions caused by heterogeneous interfaces. The oxygen surface exchange kinetics under H2/H2O atmosphere is found to be a weighted average of the intrinsic contributions of SFM and SDC phases. No evidence is found for an enhanced exchange rate at the SFM–SDC–gas triple phase boundaries (TPB). Synergetic contributions arise under CO/CO2 atmosphere, enhancing the rate of oxygen surface exchange up to a factor of 2.4. The obtained results are discussed in terms of the surface microstructure of the composites from image analysis. Overall, the results of this and our previous study confirm that the triple phase boundaries in SFM–SDC composites significantly accelerate the oxygen incorporation kinetics under oxidizing conditions, but only modestly, or even negligibly, influence the oxygen release kinetics under reducing conditions.