Some 3-glycosyl-5-aryl-2-isoxazolines were studied in the gas phase under electron ionization (EI) conditions in order to elucidate their behavior. These derivatives showed an interesting rearrangement involving opening of the heterocyclic ring with a concomitant ring closing to afford a new isoxazoline derivative. The first step of this reaction was the less common cleavage of the heterocyclic C(5)-O bond. The 5-aryl substituent was responsible for the pseudobenzylic stabilization of the new isoxazoline derivative. EI tandem mass spectrometry and EI high-resolution mass spectrometry allowed the elucidation of fragmentation pathways. Copyright 1999 John Wiley & Sons, Ltd.