Previous studies have shown that low-energy collision-induced dissociation (CID) of the important sulfonium ion metabolite S-adenosyl-L-methionine (AdoMet, m/z 399) yields five main product ions: an ion at m/z 250 arising from methionine loss; ions at m/z 102 and 298, which arise via cleavage of the γ CS bond of methionine; and ions at m/z 136 and 264, which arise via loss of protonated and neutral adenine, respectively. These metabolomics studies have, however, either totally ignored the mechanisms that govern the formation of the major product ion at m/z 250 (Gellekink H, van Oppenraaij-Emmerzaal D, van Rooij A, Struys EA, den Heijer M, Blom HJ. Clin. Chem. 2005; 51: 1487), or have proposed an oxonium ion structure that must arise via a rearrangement involving a 1,2 hydride shift (Cataldi TRI, Bianco G, Abate S, Mattia D. Rapid Commun. Mass Spectrom. 2009; 23: 3465). Here DFT calculations on a model system are used to examine potential mechanisms for the formation of the major product ion of AdoMet. These calculations suggest that a neighbouring group mechanism is preferred over a 1,2 hydride shift mechanism. Copyright © 2010 John Wiley & Sons, Ltd.