AbstractHydrogen is key to the transformation of today's energy technology toward a sustainable future without carbon dioxide emissions. Hydrogen can be produced from water using renewable or sustainable energy sources such as solar or wind power. It can buffer fluctuations between energy generation and use in all energy sectors, stationary heat, and power, as well as mobility. Safe, fast, and easy to handle solutions for storing and releasing hydrogen are essential for the implementation of hydrogen technology. Among the storage alternatives, metal hydride materials represent a safe and efficient option. For the first time, detailed investigations of the local chemical changes in a confined hydrogen storage material before and after 21 hydrogen-unloading and loading cycles are reported. The system is based on micrometer-sized reactive hydride composite (RHC) particles, namely 6Mg(NH2)2 + 9LiH + 1LiBH4, dispersed in a matrix of poly(4-methyl-1-pentene) (TPXTM). The morphological stability of the confined RHC particles during the reversible and almost complete reaction with hydrogen is visualized in detail, explaining the excellent long-term cycling stability.