Leguy, Hu, Campoy-Quiles, Alonso, Weber, Azarhoosh, vanSchilfgaarde, Weller, Bein, Nelson, Docampo, and Barnes investigated the reversible hydration of CH3NH3PbI3 in films, single crystals, and solar cells. They found that exposure of MAPI to water vapor at room temperature leads to the formation of hydrated crystal phases, which can be fully reversed upon drying. Using time-resolved XRD and ellipsometry, they observed the reversible formation of CH3NH3PbI3·H2O followed by (CH3NH3)4PbI6·2H2O upon prolonged exposure. In contrast, liquid water causes irreversible decomposition of MAPI into PbI2. The optical constants of CH3NH3PbI3·H2O formed on single crystals were determined, with a bandgap of 3.1 eV. Modeling of MAPI films exposed to moisture showed that the mono-hydrate phase forms independently of depth, suggesting rapid water transport along grain boundaries. Exposure of an unencapsulated solar cell to water vapor resulted in a 90% drop in short circuit photocurrent and 200 mV loss in open circuit potential, but these losses were fully reversed after drying. Hysteresis in current-voltage characteristics increased after dehydration, possibly due to changes in defect density and morphology. The study highlights the reversible hydration of MAPI and its implications for the stability of perovskite solar cells. The results suggest that MAPI can be reversibly hydrated and dehydrated, with the hydration process being isotropic on a macroscopic scale. The study also demonstrates that the hydration process can be reversed by drying, and that the formation of hydrate phases is crucial for understanding the stability of perovskite solar cells. The findings provide insights into the degradation mechanisms of MAPI under different environmental conditions and the importance of moisture control in the development of stable perovskite solar cells.Leguy, Hu, Campoy-Quiles, Alonso, Weber, Azarhoosh, vanSchilfgaarde, Weller, Bein, Nelson, Docampo, and Barnes investigated the reversible hydration of CH3NH3PbI3 in films, single crystals, and solar cells. They found that exposure of MAPI to water vapor at room temperature leads to the formation of hydrated crystal phases, which can be fully reversed upon drying. Using time-resolved XRD and ellipsometry, they observed the reversible formation of CH3NH3PbI3·H2O followed by (CH3NH3)4PbI6·2H2O upon prolonged exposure. In contrast, liquid water causes irreversible decomposition of MAPI into PbI2. The optical constants of CH3NH3PbI3·H2O formed on single crystals were determined, with a bandgap of 3.1 eV. Modeling of MAPI films exposed to moisture showed that the mono-hydrate phase forms independently of depth, suggesting rapid water transport along grain boundaries. Exposure of an unencapsulated solar cell to water vapor resulted in a 90% drop in short circuit photocurrent and 200 mV loss in open circuit potential, but these losses were fully reversed after drying. Hysteresis in current-voltage characteristics increased after dehydration, possibly due to changes in defect density and morphology. The study highlights the reversible hydration of MAPI and its implications for the stability of perovskite solar cells. The results suggest that MAPI can be reversibly hydrated and dehydrated, with the hydration process being isotropic on a macroscopic scale. The study also demonstrates that the hydration process can be reversed by drying, and that the formation of hydrate phases is crucial for understanding the stability of perovskite solar cells. The findings provide insights into the degradation mechanisms of MAPI under different environmental conditions and the importance of moisture control in the development of stable perovskite solar cells.