The structure of the gaseous long-lived radical cation generated upon electron ionization of trimethylphosphine oxide, (CH&PO, has been investigated by using ion-molecule reactions in a Fourier transform ion cyclotron resonance mass spectrometer. A radical cation with the connectivity of trimethylphosphine oxide is expected to react by facile electron transfer with triethylamine, pyridine and dimethyl disulfide since all these reactions are highly exothermic. However, no electron transfer reactions were observed. Instead, the radical cation transfers a proton to triethylamine and to pyridine, i.e., acts as a Brgnsted acid. Further, the radical cation abstracts CH$' from dimethyl disulfide and hence demonstrates behavior characteristic of a distonic ion with a carbon radical center. This reactivity is unprecedented for a radical cation such as (CH&P'-0' with the odd spin located at an oxygen atom. These experimental results indicate that the initially generated radical cation (CH3)3P'-O' undergoes unimolecular isomerization to (CH3)zP'(OH)CH; within a millisecond time frame. Ab initio molecular orbital calculations carried out at the unrestricted second-order Moller-Plesset (UMP2/6-31G** + ZPVE) level of theory support this conclusion by predicting that (CH&P'(OH)CH2 lies 23 kcal mol-' lower in energy than (CH&P'-0'. The energy barrier for unimolecular [1,3]-hydrogen atom migration in (CH&P'-0' is estimated to be 24 kcal mol-I. This study demonstrates that the PO moiety provides a very strong driving force for hydrogen shifts in phosphorus containing radical cations.