Gas-phase dissociation reactions of protonated amino acids-phenylalanine, tyrosine, tryptophan, and histidine-are rich and diverse. Considerable similarities exist among the four amino acids, but there are also significant differences. Facile reactions include the elimination of NH3, common to all aromatic amino acids except histidine, and the concomitant elimination of H2O and CO. Labeling experiments with deuteriums show considerable H/D scrambling prior to dissociation involving N-H O-H and C-H (both aliphatic and aromatic hydrogens). Mechanisms of this scrambling are proposed. At higher collision energies, eliminations of H2O, CO, CO2, and CH2CO occur after that of NH3. Similarly, eliminations of HCN, HCNH2, and NH3 occur after that of H2O and CO. The elimination of CH2CO is preceded by migration of the hydroxyl ion from the carboxylic group to the exocyclic carbon on the side chain. Aromatic amino acids, with the exception of tyrosine, were observed to yield cationic radical fragments by eliminating small radicals, including H-., CH3., and NH=CH..