The reaction of lithium hydride (LiH) powder with pure water vapor (H2O and D2O) was studied by thermogravimetry and in situ infrared spectroscopy at 298 K over a large pressure range. The mean particle size of LiH is around 27 mu m. At very low pressure, the hydrolysis starts with the formation of lithium oxide (Li2O). Then, both Li2O and lithium hydroxide (LiOH) are formed on increasing pressure, thus, creating a Li2O/LiOH bilayer. The reaction takes place through the consumption of LiH and the formation of Li2O at the LiH/Li2O interface and through the consumption of Li2O and the formation of LiOH at the Li2O/LiOH interface. Above 10 hPa, only the monohydrate LiOH center dot H2O is formed. This hydration reaction of LiOH into LiOH center dot H2O occurs at a lower pressure (8 hPa) after the first hydration-dehydration cycle. The hydrolysis mechanism proposed in this paper suggests that the diffusion of ionic species across the intermediate Li2O layer is the rate limiting step of the reaction. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.