Compact interseasonal thermal storage is a key enabler for renewable heating. A promising approach is the liquid absorption process, extended with absorbent and absorbate storage. In sorption heat storage, the conventional parameters—temperature gain and power density, governing the sorption heat pump process—are extended by the parameter energy density. This opens up new challenges for heat and mass exchanger design, demanding a detailed understanding of the fundamental mass transport process under technically relevant constraints. Toward this objective, investigation in the water mass transport in a static aqueous sodium hydroxide thin film at application‐specific temperature and pressure using temporally and spatially resolved Raman spectroscopy is performed. Based on the measured concentration gradient in the film, it is determined that the mass transport in the film and not the liquid–gas interface is limiting.