Hydration of the ammonium form of the solid acid catalyst ZSM-5 is investigated by applying a technique developed recently for in situ solid-state NMR studies of adsorption processes. From high-resolution solid-state H-1 NMR spectra recorded as a function of time during the hydration process, insights are established concerning the nature of the interaction between the adsorbed water molecules and the ammonium cations in the ZSM-5 host material. In particular, quantitative information is established regarding the clustering of water molecules during the hydration process, and indicates the preferential formation of clusters comprising two water molecules associated with each hydrated ammonium cation.