AbstractThe fragmentation pathways of ten 4‐substituted quinolizid‐2‐ones were established using exact mass measurement, deuterium labelling and detection of metastable transitions. The base peak for most ketones was formed by cleavage of the C‐2C‐3 bond (α to the carbonyl group) and the benzylic C‐4N bond. The decompositions examined were compared with those of the Nuphar alkaloids.