Solvent structures that surround active sites reorganize during catalysis and influence the stability of surface intermediates. Within zeolite pores, H2O molecules form hydrogen-bonded structures that differ substantially from bulk H2O. Here, we show by spectroscopic measurements and molecular dynamics simulations that H2O molecules form bulk-like three-dimensional structures within 1.3 nm cages, whereas H2O molecules coalesce into oligomeric one-dimensional chains when the pore diameter falls below 0.65 nm. The differences between these solvent structure motifs provide opportunities to manipulate enthalpy–entropy compensation relationships and greatly increase the rates of catalysis. We describe how the reorganization of these pore-size-dependent H2O structures during alkene epoxidation catalysis gives rise to entropy gains that increase the turnover rates by up to 400-fold. Collectively, this work shows that solvent molecules form distinct structures with a highly correlated motion within microporous environments, and the reorganization of these structures may be controlled to confer stability to the desired reactive intermediates.