The structures of hydrogen bonded H 2 O solvent clusters depend sensitively on the shape and size of the surrounding void. Within 1.3 nm cavities, H 2 O molecules coalesce into three-dimensional clusters that resemble bulk liquid. In contrast, H 2 O within sub-nanometer pores form short largely one-dimensional chains of molecules that exist throughout the pore structure and possess cross-linking hydrogen bonds primarily at intersections between pores. During liquid-phase catalysis, these H 2 O structures must reorganize to accommodate the formation of reactive surface intermediates which leads to disproportionate increases in entropy that outweigh the enthalpic penalty for disrupting these confined solvent molecules. These phenomena likely affect a wide range of catalytic reactions that proceed within zeolite pores filled with aqueous solvents, and these reactions should therefore follow the enthalpy-entropy compensation relationships described here (Table ).