Protonated glycine is produced in the gas phase by fast atom bombardment or chemical ionization (with CH5+), and its structures and unimolecular reactions are investigated by a combination of tandem mass spectrometry methods, including metastable ion (MI) characteristics, collisionally activated dissociation (CAD), and neutralization-reionization mass spectrometry (NRMS). The major fragmentations of [H2NCH2COOH]H+ are elimination of CO to form the ion-molecule complex H2O...(H2N)-H-+=CH2 and consecutive cleavages of H2O + CO to yield the immonium ion (H2N)-H-+=CH2. Other important reactions involve the losses of H-., H-. + H2O, H2N., H2N. + H2O, (COOH)-C-., and CH5N. Due to the facile H+ migration possible in [H2NCH2COOH]H+, the precise site of protonation cannot unequivocally be pinpointed based on the unimolecular reactions of this ion. More definitive information is obtained from the chemistry of neutralized [H2NCH2COOH]H+. Analysis of this neutral by NRMS reveals that it consists of (H3NCH2COOH)-H-. and H2NCH2C.(OH)(2) at the moment of neutralization. Thus, the original protonation process must have created both tautomers. Once formed, the hypervalent ammonium radical (H3NCH2COOH)-H-. (neutral counterpart of N-protonated glycine) dissociates completely to H-. + H2NCH2COOH and H3NCH2 (or H2NCH3) + (COOH)-C-.. On the other hand, the dihydroxy alkyl radical H2NCH2C.(OH)(2) (neutral counterpart of O-protonated glycine) undergoes several distinctive fragmentations, e.g., to H2N. + C2H4O2 and H2NCH2. + CO + H2O.