The structure and formation of [cI&O]" ions generated from phenylcyclopropylcarbiiol and l-phenyl-lhydroxymethylcyclopropane upon electron impact, have been studied using kinetic energy release measurements, by determination of ionization and appearance energies and by collisional activation. It is shown that the non-decomposing [C3&0]" ions have exclusively the structure of the enol ion of phenylacetaldehyde, although it is less stable than the enol ion of acetophenone by about 45 kJ mol-l. This has been interpreted as an indication that the [C3&0]" ions from phenylcyclopropylcarbinol are formed by an attack of either the phenyl ring or the hydroxyl group upon the C-1-C-2 (or C-1-C-3) bond of the cyclopropane ring under a simultaneous expulsion of ethene and migration of the attacking group to the C-1 position. The [C&,O]+* ion from l-phenyl-l-(hydroxymethyl)cyclopropane is formed by opening of the eyelopropane ring via a benzylic cleavage. A k h e t i d y controlled hydrogen shift in the resulting ring opened ion prior to or during ethene loss then leads to the formation of [C&&O]" ions which have the structure of the enol ion of phenylacetaldehyde.