Abstract Methods are described for the unequivocal identification of the acetyl, [CH 3 $$ \mathop {\rm C}\limits^{\rm + } $$ O] ( a ), 1‐hydroxyvinyl, [CH 2 $$ \mathop {\rm C}\limits^{\rm + } $$ OH] ( b ), and oxiranyl, ( d ), cations. They involve the careful examination of metastable peak intensities and shapes and collision induced processes at very low, high and intermediate collision gas pressures. It will be shown that each [C 2 H 3 O] + ion produces a unique metastable peak for the fragmentation [C 2 H 3 O] + → [CH 3 ] + +CO, each appropriately relating to different [C 2 H 3 O] + structures. [CH 3 $$ \mathop {\rm C}\limits^{\rm + } $$ O] ions do not interconvert with any of the other [C 2 H 3 O] + ions prior to loss of CO, but deuterium and 13 C labelling experiments established that [CH 2 $$ \mathop {\rm C}\limits^{\rm + } $$ OH] ( b ) rearranges via a 1,2‐H shift into energy‐rich leading to the loss of positional identity of the carbon atoms in ions ( b ). Fragmentation of b to [CH 3 ] + +CO has a high activation energy, c. 400 kJ mol −1 . On the other hand, , generated at its threshold from a suitable precursor molecule, does not rearrange into [CH 2 $$ \mathop {\rm C}\limits^{\rm + } $$ OH], but undergoes a slow isomerization into [CH 3 $$ \mathop {\rm C}\limits^{\rm + } $$ O] via [CH 2 $$ \mathop {\rm C}\limits^{\rm + } $$ HO]. Interpretation of results rests in part upon recent ab initio calculations. The methods described in this paper permit the identification of reactions that have hitherto lain unsuspected: for example, many of the ionized molecules of type CH 3 COR examined in this work produce [CH 2 $$ \mathop {\rm C}\limits^{\rm + } $$ OH] ions in addition to [CH 3 $$ \mathop {\rm C}\limits^{\rm + } $$ O] showing that some enolization takes place prior to fragmentation. Furthermore, ionized ethanol generates a , b and d ions. We have also applied the methods for identification of daughter ions in systems of current interest. The loss of OH ˙ from [CH 3 COOD] +˙ generates only [CH 2 $$ \mathop {\rm C}\limits^{\rm + } $$ OD]. Elimination of CH 3 ˙ from the enol of acetone radical cation most probably generates only [CH 3 $$ \mathop {\rm C}\limits^{\rm + } $$ O] ions, confirming the earlier proposal for non‐ergodic behaviour of this system. We stress, however, that until all stable isomeric species (such as [CH 3 $$ \mathop {\rm O}\limits^{\rm + } $$ C:]) have been experimentally identified, the hypothesis of incompletely randomized energy should be used with reserve.