Frontispiece: In Situ Solid-State Reactions Monitored by X-ray Absorption Spectroscopy: Temperature-Induced Proton Transfer Leads to Chemical Shifts.

X ‐ ray Absorption Spectroscopy In many chemical and biological processes, proton transfer plays a crucial role by impacting the electronic structure and functional properties of materials. Direct evidence of a proton transfer within the solid state is captured by chemical shifts in X ‐ ray absorption spectroscopy (XAS) in the Communication by J. S. Stevens et al. on page 15600 ff. Heating of the squaric acid/4,4′ ‐ bipyridine system leads to a dramatic, rapid phase and colour transition through a cascading Brønsted acid ‐ base reaction, showcasing XAS as an unambiguous identifier for proton transfer without the need for a priori information and the ability to monitor organic reactions in situ. [ABSTRACT FROM AUTHOR]