An iron-based oxygen carrier can convert natural gas into chemicals (syngas or hydrogen) with controlled CO2 emission in a redox process. Mixed ionic-electronic conductor (MIEC)-supported iron oxides have shown high catalytic activity by facilitating inward 0 anion diffusion. However, their durability has been tested under limited conditions, and key factors affecting the degradation of MIEC-supported iron oxides have rarely been identified. In this work, we find that the inherent redox stability and electronic conductivity of the support material are decisive properties that determine the redox stability and activity of iron oxide/MIEC composites, such as perovskite-type La0.8Sr0.2FeO3-delta and fluorite-type Ce0.9Gd0.1O2-delta, over 100 redox cycles with the redox pair Fe - Fe2O3 at 900 degrees C. The low redox stability of the Fe2O3/La0.8Sr0.2FeO3-delta composite oxygen carrier is closely related to that of La0.8Sr0.2FeO3-delta and the extreme redox environment. The increased electronic conductivity of Ce0.9Gd0.1O2-delta under reducing conditions enhances the reaction rate. However, the low electronic conductivity of Ce0.9Gd0.1O2-delta under oxidizing conditions (5 x 10(-4) S/cm at 750 degrees C) progressively promotes the formation of iron oxide product layer, resulting in low syngas selectivity (H-2/CO > 2). This work helps design and select a compatible and commercially viable MIEC-supported iron oxide.