The MS/MS spectrum of the metastable molecular ions of dimethyl terephthalate I features an unexpected, quite intense, loss of a methyl radical. Using a combination of mass spectrometry techniques, it is shown that this process consists of a pseudo simple cleavage reaction isomerizing, in the first step, one of the two methoxycarbonyl substituents into a distonic ArC+(-OH)-OCH2 (center dot) (Ar = CH3O2C-C6H4-) connectivity. The final product is established to be protonated 5-carboxyphthalide. The overall mechanism of the methyl loss includes a sequence of five steps as supported by quantum chemical (DFT) calculations. (C) 2006 Elsevier B.V. All rights reserved.