The effect of ionization on the relative stabilities of the four lowest conformers of glycine and on the intramolecular proton transfer process has been studied using density functional and MP2 methods. Single-point calculations at the CCSD(T) level have also been performed. The energy ordering of the radical cations differs from that observed for the neutral conformers, mainly due to the changes in the basic and acid character of the NH2 group upon ionization. Ionization favors the intramolecular proton-transfer process. For the ground ionic state, 2A′, both reactant and product have similar energies, the energy barrier being about 9.0 kcal/mol. For the first excited state, the proton transfer appears to be spontaneous. However, in both cases, the final product is a distonic [NH3+-CH2-COO-] radical cation. This is in contrast to what is observed for neutral glycine, for which the zwitterionic structure is not stable in the gas phase. © 2000 American Chemical Society.