AbstractDetermination of a reaction pathway is an important issue for the optimization of reactions. However, reactions in solid‐state compounds have remained poorly understood because of their complexity and technical limitations. Here, using state‐of‐the‐art high‐speed time‐resolved synchrotron X‐ray techniques, the topochemical solid‐gas reduction mechanisms in layered perovskite Sr3Fe2O7−δ(fromδ∼ 0.4 toδ= 1.0), which is promising for an environmental catalyst material is revealed. Pristine Sr3Fe2O7−δshows a gradual single‐phase structural evolution during reduction, indicating that the reaction continuously proceeds through thermodynamically stable phases. In contrast, a nonequilibrium dynamically‐disordered phase emerges a few seconds before a first‐order transition during the reduction of a Pd‐loaded sample. This drastic change in the reaction pathway can be explained by a change in the rate‐determining step. The synchrotron X‐ray technique can be applied to various solid‐gas reactions and provides an opportunity for gaining a better understanding and optimizing reactions in solid‐state compounds.