For the mechanisms by which the oxygen gets incorporated in a dual-phase composite system, three hypotheses, i.e. cation inter-diffusion, spillover type and self-cleaning of the perovskite-structured phase, have been provided in the literature. However, experimentally a consensus on the most likely mechanism is yet to be reached. In this work, a specially fused sample of the lanthanum strontium chromium ferrite (LSCrF)-scandia/ceria-stabilised zirconia (ScCeSZ) dual-phase material was investigated. Among the three potential mechanisms, no obvious cation inter-diffusion was firstly observed. A cleaner surface of the ScCeSZ phase was confirmed in the fused sample than in the isolated ScCeSZ single-phase sample while impurity layers were clearly observed on the LSCrF surface, suggesting the cleaning effect from the perovskite. However, more evidence implies that the cleaning effect is not the only reason for the synergistic effects between these two phases. Observations via SIMS analysis lend strong support to the 'spillover-type' mechanism as the oxygen isotopic fraction on the surface of the ScCeSZ increased compared to the isolated single-phase and as the distance to the heterojunction increases, the oxygen isotopic fraction decreases. Moreover, oxygen depleted layers were clearly seen on the top layers of the LSCrF surface which may be associated with the higher oxygen diffusivity in the surface/sub-surface layers, oxygen grain boundary fast diffusion and the impurities on the perovskite phase. For this sample, a combination of 'spillover' and 'self-cleaning' type mechanisms is suggested to be the potential possibility while the contribution from the cation inter-diffusion for this specific sample is proven to be low.