The mass spectra of some lJ,rl-substituted azetidin-2-ones are measured. Fragmentation pathways are confirmed by use of both high-resolution and mass-analysed ion kinetic energy. They are consistent with the possible 12 + 2)-cycloadditions through which the azetidin-2-ones may be synthesized. More attention has to be paid to further fragmentations of the main fragments in assessing the preferred fragmentation mode. Influence of side-chain suhstituents is discussed.