 The reaction pathways of a layered perovskite, senior 3-phate 207, delta, delta ranging from 0.4 to 1.0, were studied using state-of-the-art high-speed time-resolved synchrotron x-ray techniques. It was found that the reaction progresses through a gradual single-phase structural evolution when pristine senior 3-phate 207, delta, is reduced, while a non-equilibrium dynamically disordered phase emerged at a certain point prior to a first-order transition in a PD-loaded sample. This drastic change in the reaction pathway could be explained by a change in the rate-determining step. Synchrotron x-ray techniques are applicable to various solid gas reactions and provide an opportunity for gaining a better understanding and optimizing reactions in solid state compounds. This article was authored by Takafumi Yamamoto, Shogo Kawaguchi, Taiki Kosuj, and others.