The remarkable unimolecular chemistry of the acetol cation radical, CH3C(=O)CH2OH.+ (4), has been studied by a wide range of mass spectrometric experiments. A detailed analysis of the D-labeled isotopomers of 4, CH3C(=O)CD2OH.+ and CH3C(=O)CH2OD.+, shows that the hydrogen-bridged ion [CH3C=O...H...O=CH2](.+) (3a), a key intermediate on the (related) methyl acetate (1) hypersurface, does not participate in the low-energy rearrangement reaction : 4 --> CH3CHOH+ + HC=O-.. Integration of the experimental information with the results of ab initio molecular orbital calculations (executed at the UMP3/6-31G*//4-31G+ZPVE level of theory and for the key ions 4, 3a, and 3b also at the UMP3/6-31G**//6-31G** and UMP3/6-31G*//UMP2/6-31G* levels) provides evidence that the loss of HC=O-. from 4 involves isomerization into the ion-dipole complexes CH3C(=O)(+)...O(H)-CH2. (4a) and CH3C(=O)H-.+...O=CH2 (3b). It is further proposed that the hypersurfaces of ionized acetol and methyl acetate have no common features. The calculations also predict that another stable O...H...O hydrogen-bridged isomer, [CH2=C(H)O...H...O=CH2](.+) (6) does not communicate with 4 or 3a. This isomer was independently generated and characterized : at elevated energies it behaves as 9 vinyl alcohol ion-formaldehyde complex, and the experiments confirm that it does not communicate with either the acetol or the methyl acetate hypersurface.