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Hydrogen–deuterium exchange reactions of 16 small rigid anions were examined using D 2 O, CH 3 OD, CH 3 CO 2 D, and (CF 3 ) 2 C(CH 3 )OD. Bimolecular rate constants and extensive computations are reported. These deuterated reagents were used because D 2 O and CH 3 OD are weak acids that cannot react via sequential deuteron/proton transfers (i.e., the conventional H/D exchange mechanism of DePuy et al.) with the selected anions, and they provide a means to probe alternative pathways. The relative acidity of CH 3 CO 2 D and (CF 3 ) 2 C(CH 3 )OD was measured (ΔΔ H acid ° = 1.1 ± 0.8 kcal mol −1 ) and their thermodynamic similarity but structural differences (i.e., the presence of a C O in the carboxylic acid but not the alcohol) were exploited to systematically investigate flip-flop and relay mechanisms. This has led to a number of generalizations relating to the occurrence of these pathways. [ABSTRACT FROM AUTHOR]