The oxygen incorporation kinetics of Sr2Fe1.5Mo0.5O6 d–Sm0.2Ce0.8O1.9 (SFM–SDC) dual-phase composites has been investigated, at 750 C, as a function of SDC phase volume fraction using electrical conductivity relaxation. It is shown that the oxygen re-equilibration kinetics in the range of oxygen partial pressure (pO2) from 0.01 to 1 atm is limited by the surface exchange rate. The effective surface exchange coefficient of the composites is found to increase profoundly upon increasing the phase volume fraction of the oxide electrolyte phase SDC. The results are interpreted to reflect the synergistic oxygen incorporation at the SFM–SDC–gas triple phase boundaries (TPBs), which occurs in addition to the direct incorporation via the surface of the perovskite mixed conductor SFM. Already at a SDC phase volume fraction of 0.105, the uptake of oxygen via the synergistic TPB route (referred to as route III), following a step change in the surrounding pO2, comprises more than 75% of the overall uptake of oxygen by the composite. It is further concluded that under the conditions of the experiments the twophase SFM–SDC boundaries allow for a facile exchange of oxygen ions between both involved phases.