The gas phase is an idealized laboratory for the study of protein structure, from which it is possible to examine stable and transient forms of mass-selected ions in the absence of bulk solvent. With ion mobility-mass spectrometry (IM-MS) apparatus built to operate at both cryogenic and elevated temperatures, we have examined conformational transitions that occur to the monomeric proteins: ubiquitin, lysozyme, and α-synuclein as a function of temperature and in source activation.