Hydrogen/deuterium (H/D) exchange reactions of fluorophenyl and difluorophenyl anions (C6H4F−, o-C6H3F 2 − , m-C6H3F 2 − , p-C6H3F 2 − ) have been studied using the flowing afterglow-selected ion flow tube technique. The C6H4F− anion exchanges all hydrogens for deuterium upon reaction with D2O. The difluorophenyl anions o-, m-, and p-C6H3F 2 − exchange three, two, and one hydrogen, respectively, with D2O, whereas they undergo one, two, and three H/D exchanges, respectively, with CH3OD. The structures of the anions and the isotope exchange dynamics within the intermediate ion-dipole complexes are discussed using ab initio molecular orbital calculations. Calculated values for the proton affinities of the most stable anions are 385.2, 378.0, 371.9, and 378.2 kcal/mol for C6H4F−, o-C6H3F 2 − , m-C6H3F 2 − , and p-C6H3F 2 − , respectively, in excellent agreement (within 2 kcal/mol) with the previous experimental values for the acidities of the corresponding fluorobenzenes. The H/D exchange results are explained by the energy differences of the intermediate DO− and CH3O− species within the ion-dipole complexes; CH3O− is mobile within the “hot” intermediate complex, whereas DO− is nearly “frozen” within the complex and cannot migrate across the barriers caused by the fluorine atoms or by the π electrons.