The study investigates the reversible hydration of methylammonium lead iodide (MAPI) perovskite in films, single crystals, and solar cells. The researchers found that MAPI forms hydrated crystal phases when exposed to water vapor at room temperature, which can be fully reversed by drying. Time-resolved X-ray diffraction and ellipsometry were used to monitor the hydration and dehydration processes. The reversible formation of CH₃NH₃PbI₃•H₂O and (CH₃NH₃)₄PbI₆•2H₂O was observed in thin films, single crystals, and solar cells. The presence of liquid water, however, leads to the irreversible decomposition of MAPI into PbI₂. The hydration process was found to be isotropic within the granular thin films, suggesting rapid water molecule diffusion along grain boundaries. The reversible hydration has implications for the stability of MAPI-based solar cells, as partial hydration can cause a significant drop in performance, which can be recovered by drying. The study provides insights into the degradation mechanisms of MAPI perovskite and offers strategies to improve device stability.The study investigates the reversible hydration of methylammonium lead iodide (MAPI) perovskite in films, single crystals, and solar cells. The researchers found that MAPI forms hydrated crystal phases when exposed to water vapor at room temperature, which can be fully reversed by drying. Time-resolved X-ray diffraction and ellipsometry were used to monitor the hydration and dehydration processes. The reversible formation of CH₃NH₃PbI₃•H₂O and (CH₃NH₃)₄PbI₆•2H₂O was observed in thin films, single crystals, and solar cells. The presence of liquid water, however, leads to the irreversible decomposition of MAPI into PbI₂. The hydration process was found to be isotropic within the granular thin films, suggesting rapid water molecule diffusion along grain boundaries. The reversible hydration has implications for the stability of MAPI-based solar cells, as partial hydration can cause a significant drop in performance, which can be recovered by drying. The study provides insights into the degradation mechanisms of MAPI perovskite and offers strategies to improve device stability.