The identification and understanding of structure-activity relationships is vital for rational catalyst design. A kinetic study of the hydrogen-deuterium exchange reaction of cyclohexanone in aqueous solution, as catalyzed by proline derivatives, has revealed valuable structure-activity relationships. In phosphate-buffered solution, cis-4-fluoroproline is more active than the trans isomer, a distinction that appears to originate from a destabilizing interaction between the fluorine atom and phosphate anion during general acid-catalyzed dehydration of the carbinolamine intermediate. trans-4-Ammoniumprolines are exceptionally active catalysts owing to favorable Coulombic interactions involving the ammonium group and the alkoxide moiety formed upon 1,2-addition of the proline derivative to the ketone. These results could be used for the optimization of proline catalysts, especially in transformations where the formation of the putative iminium ion is rate-limiting.