AbstractAn [M  31]+ion was a prominent fragment in the mass spectra of threeortho‐methoxy‐phenyl‐2‐propanone oximes and is shown to be due to the expulsion of a methoxyl radical from the molecular ion as a result of anortho‐effect. In contrast, an [M  31]+ion was absent from the spectra of a structurally related ketone and a hydroxylamine, and was not observed in the spectra ofmeta‐ andpara‐methoxyphenyl