Anils, N-salicylideneanilines, display thermochromism, which is thought to involve an intramolecular proton shift, but to-date the structural changes postulated to occur during this transformation have not been observed by X-ray diffraction. We have synthesized 4 anils varying only in the halogen substituent, and X-ray studies have shown that each has different populations of the enol and cis-keto forms. One of these clearly shows shifting between forms with temperature, and variable temperature single-crystal neutron diffraction is essential to accurately locate hydrogen atoms and determine split-site hydrogen occupancies. Subtle shifts between the two forms with temperature, inaccessible by X-ray diffraction, will be monitored. Knowledge of such solid-state mechanisms, including thermochromism in anils, is essential to design materials suitable for solid-state switching applications.