The hydration of K2SO4 . 2MgSO(4) as a function of water addition was examined by pore-size distribution, thermo-gravimetric analysis/differential thermal analysis and nuclear magnetic resonance (NMR) spectroscopy. Thermal studies indicated that the dehydration reactions occur within three distinct temperature regimes which correspond to different degree of hydrogen bonded water or water in different pore-size environments. Solid NMR studies resolved three different proton peaks at each water loading, that represent different chemical environments around water, and also the different range of hydrogen bonding strengths. Therefore, the different chemical environments associated with pore size around water are key factors influencing the NMR spectra. Both thermal and NMR studies suggested that pore size affects the molecular interactions between water and water, and water and the substrate. Also X-ray diffraction studies identified the presence of three phases existing in the hydrated products after the hydration reaction.