Amide hydrogen deuterium exchange is widely used to probe conformations of gas-phase biomacromolecular ions. Meaningful interpretation of such experiments depends on relating the experimentally determined H/D exchange rate constants to specific exchangeable hydrogen sites on the molecule. Here, we present the first definitive assignment of H/D exchange rate constants for the N-terminal ammonium, amide hydrogen, and carboxyl hydrogens in diglycine, based on comparison of site-specific gas-phase H/D exchange rate constants for diglycine (GlyGly) and its selectively methylated homologues (GlySar, GlyGly-OMe, and GlySar-OMe). The present method may be extended to identification of site-specific H/D exchange rate constants for other dipeptides, to provide insight into H/D exchange reaction mechanisms.