Dissociation kinetics of the K+ loss reaction of three potassiated tertiary amino acids (Scheme 1) were studied by infrared multiple photon dissociation (IRMPD) in a Fourier transform ion cyclotron resonance (FT ICR)-MS instrument. The aim of the study was to probe if a kinetic study by IRMPD can yield useful information oil the ion structure of the precursor ion species. The measured activation energy values determined by IRMPD are related to the potassium affinity, Delta AH(K+), of N-methyl proline determined by threshold collision-induced dissociation experiments. By appropriate scaling with this reference value, the experimentally determined IRMPD activation energy values for the K+ loss are transformed into respective potassium affinities, Delta HK+ These values match die calculated potassium affinity values for salt bridge (SB) structures, Delta HK+SB, substantially better than those for canonical structures with a single formal charge site (charge solvation (CS)), thereby allowing structure identification. This conclusion is consistent with other spectroscopic data, which yielded unambiguous evidence of these tertiary amino acids adopting SB structures in the gas phase. This study demonstrates that IRMPD can be applied to determine individual ion structures in the gas phase, given that adequate reference values are available for proper scaling.