Here, we characterize the structure of H 2 O contained within FAU, BEA, MFI, and CDO zeolites of varying silanol density using a combination of spectroscopic and kinetic methods and molecular dynamics simulations to illustrate the impact of pore topology and polarity. We elucidate how the reorganization of H 2 O solvent structures during epoxidation catalysis changes the free energies of reactive surface intermediates and facilitates reaction. H 2 O molecules form bulk-like structures within the 1.3 nm supercages of FAU zeolites. In contrast, H 2 O molecules coalesce into confined one-dimensional chains as the characteristic pore dimension decreases from the 1.3 to 0.45 nm. Kinetic measurements of 1-alkene epoxidation over Ti-containing zeolites provide a complementary method to evince the effects of confined H 2 O molecules on the stability of species relevant for catalysis. Confined H 2 O molecules dynamically restructure to accommodate the formation of reactive intermediates, and these processes produce large differences (greater than 400-fold) in measured epoxidation rates. Collectively, these data and interpretations demonstrate how solvent molecules may form complex structures within confining environments, whose reorganization during catalysis may have a large impact on the stability of surface intermediates and measured rates and selectivities. These phenomena likely extend to other classes of microporous materials (e.g., carbon nanotubes, metal-organic frameworks, silico-alumino phosphates) and polar and protic solvents (e.g., alcohols, lactones), which present new opportunities to understand and engineer solid-liquid interfaces.