Zeolites confine active sites within void spaces of molecular dimension. The size and shape of these voids can be tuned by changing framework topology, which can influence catalytic reactivity and selectivity via coupled reaction-transport phenomena that exploit differences in transport properties among reactants and/or products that differ in size and shape. The polarity and solvating properties of intrazeolite void environments can be tuned by changing chemical composition and structure, ranging from hydrophobic defect-free pure-silica surfaces to silica surfaces containing hydrophilic defect sites and/or heteroatoms. Here, we discuss how the polarity of zeolite voids influences catalytic reactivity and selectivity via the partitioning of reactant, product, and solvent molecules between intrazeolitic locations and external fluid phases. These findings provide a conceptual basis for developing selective catalytic processes in aqueous media using hydrophobic zeolites that are able to adsorb organic reactants while excluding liquid water from internal void spaces. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 3349-3358, 2013