Abstract: A cyclization of N-[2-(benzoyloxy)phenyl]-benzamide, accompanied by the elimination of a molecule of benzoic acid, takes place in a mass spectrometer upon protonation by fast atom bombardment (FAB) or electrospray ionization (ESI). The elimination of benzoic acid from the [M + H]+ yields protonated 2-phenylbenzoxazole by a process that is analogous to the acid catalyzed cyclization of N-[2-(benzoyloxy)phenyl]-benzamide in solution. A similar elimination of benzoic acid occurs from the molecular radical cation produced by electron ionization (EI). The proposed cyclization and elimination of benzoic acid are supported by accurate mass measurements, product-ion spectra in tandem mass spectrometry (MS/MS), mass spectra of reference compounds, isotopic labeling, and molecular modeling by density functional theory. International Journal of Mass Spectrometry Volumes 249-250, 1 March 2006, Pages 21-30 Chava Lifshitz Memorial Issue