Treatment of homo- and heterocyclic aromatic substrates with basic deuterium oxide under near- or supercritical conditions results in rapid base-catalyzed hydrogen-deuterium exchange (HDE) in aromatic and benzylic positions. It has been postulated that HDE follows a simple deprotonation-reprotonation mechanism, but little evidence has been provided to date. This study correlates experimentally observed proton exchanges in n-butylbenzene with ab initio calculations of the acidities and potential energy (PE) profiles. In addition to providing further support for carbanion intermediacy in HDE, these results offer new insights into substrate acidities in near- and supercritical aqueous media and the optimal conditions required for their isotope exchange.