Gas-phase ion chemistry is an area in mass spectrometry that has received much research interest since the mid fifties of the last century. Although the focus of mass spectrometric research has shifted the last twenty years largely to life science studies, including proteomics, genomics and metabolomics, there are still several groups in the world active in gas-phase ion chemistry of both positive and negative ions, either unimolecularly and/or bimolecularly. In this tutorial lecture the formation and determination of tautomeric ion structures and intra-ionic catalyzed tautomerization in the gas phase will be discussed. In addition, an example of formation of different tautomeric structures in protic and aprotic solvents under electrospray ionization conditions will be given, as established by gas-phase infrared multiphoton dissociation spectroscopy. This will be followed by presenting an example of time-resolved MS/MS which enables to identify the structure of an ion, generated at a particular molecular ion lifetime. At the end of the lecture the power of ion mobility will be shown in elucidating the mechanism of epimerization of bis-Tröger bases having chiral nitrogen centers.