The noncovalent tetrameric active forms of avidin, concanavalin A (Con A), and adult human hemoglobin (HbA(0)) can be observed intact in the gas phase by electrospray ionization mass spectrometry (ESI-MS). The atmosphere vacuum ESI interface conditions strongly influence the retention of these weak noncovalent solution associations into the gas phase, as well as the average extent of charging for the subunits upon dissociation. The known solution pH dependence of the dimer-tetramer equilibrium of Con A was observed by ESI-MS, and the intact heterodimeric and -tetrameric active forms of adult human HbA(0), (alpha beta) and (alpha beta)(2), with the prosthetic heme groups could also be characterized by ESI-MS. Under harsher interface conditions a species corresponding to a trimer was observed for each of the proteins, a species not known to be formed under physiological conditions. Differences in the relative stabilities of these tetrameric proteins, formed from the known solution structures, are also qualitatively consistent with the gas-phase stability observed with ESI-MS by adjusting the atmosphere-vacuum interface conditions. The hemoglobin tetramer was found to be less stable in the gas phase than either the Con A or avidin tetramer, consistent with solution dissociation constants.