Deprotonation of tyrosine in the gas phase was found to occur preferentially at the phenolic site, and the conjugate base consists of a 70:30 mixture of phenoxide and carboxylate anions at equilibrium. This result was established by developing a chemical probe for differentiating these two isomers, and the presence of both ions was confirmed by photoelectron spectroscopy. Equilibrium acidity measurements on tyrosine indicated that δG̊ acid = 332.5 ± 1.5 kcal mol- 1 and δH̊ acid = 340.7 ± 1.5 kcal mol -1. Photoelectron spectra yielded adiabatic electron detachment energies of 2.70 ± 0.05 and 3.55 ± 0.10 eV for the phenoxide and carboxylate anions, respectively. The H/D exchange behavior of deprotonated tyrosine was examined using three different alcohols (CF 3CH 2OD, C 6H 5CH 2OD, and CH 3CH 2OD), and incorporation of up to three deuterium atoms was observed. Two pathways are proposed to account for these results, and all of the experimental findings are supplemented with B3LYP/aug-cc-pVDZ and G3B3 calculations. In addition, it was found that electrospray ionization of tyrosine from a 3:1 (v/v) CH 3OH/H 2O solution using a commercial source produces a deprotonated [M-H] - anion with the gas-phase equilibrium composition rather than the structure of the ion that exists in aqueous media. Electrospray ionization from acetonitrile, however, leads largely to the liquid-phase (carboxylate) structure. A control molecule, p-hydroxybenzoic acid, was found to behave in a similar manner. Thus, the electrospray conditions that are employed for the analysis of a compound can alter the isomeric composition of the resulting anion.