AbstractElimination of CO gives rise to prominent peaks in the mass spectra ofcis‐1,2,3,6‐tetrahydrophthalic anhydride (Ia) and its 4,5‐dimethyl analogue which are almost absent in the correspondingtrans‐isomers. The striking difference in the fragmentation of the isomers shows that practically no rearrangement in the molecular ion occurs prior to this fragmentation. In this case a high degree of stereospecificity is observed, although hydrogen migration is apparently not involved in the