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Owing to the unclear temporal and spatial variations of axial solid conversion in a packed bed using iron (III) oxide as an oxygen carrier, we directly observe these variations by means of a sub-layer approach. The results indicate that the behaviour of the multiple reaction fronts during iron (III) oxide reduction by CO or H2 within a packed bed for chemical looping water splitting (CLWS) is strongly dependent on the reaction temperature. When the reaction temperature is lower than the merging temperature, three reaction fronts, i.e., Fe2O3-Fe3O4, Fe3O4-Fe0.947O and Fe0.947O-Fe, and three product zones, i.e., Fe3O4, Fe0.947O and Fe, will appear in the packed bed. In contrast, when the reaction temperature is higher than the merging temperature, the Fe2O3-Fe3O4 and Fe3O4-Fe0.947O fronts merge, leading to the disappearance of the Fe3O4 zone. As a result, only the Fe2O3-Fe0.947O and Fe0.947O-Fe fronts, as well as Fe0.947O and Fe zones will appear in the packed bed. These reduction behaviours are verified by two breakthrough curves, one for T < Tm and one for T > Tm, from the thermodynamically controlled reduction of iron (III) oxide in the packed bed. The reaction front movement model, which is proposed based on the reduction behaviour, can be used to determine the maximum solid conversion of the reduction step, i.e., the thermodynamic limitation of the reduction step, in the packed bed CLWS. The maximum solid conversion can reach 0.409 for the CO case and 0.554 for the H2 case. The first discovery of both the behaviours of the reaction fronts movement and the thermodynamic limitations of the reduction step standardizes the criteria for both the oxygen carrier evaluation and the optimization of the operating conditions and provides theoretical support for scaling up the packed bed and developing new technology for packed bed CLWS

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