Ion-molecule reactions and collision-activated dissociation experiments were used to examine the structure of the long-lived, low-energy molecular ion of hydroxyacetone [CH3C(O)CH2OH].+ (1) in a dual-cell Fourier-transform ion cyclotron resonance mass spectrometer (Fr-ICR). It is demonstrated that a significant proportion of the nonfragmenting ions have a noncovalent, hydrogen-bridged structure, most likely [CH3C=O...H...Os=CH2].+ (2). This hydrogen-bridged ion undergoes reactions which are unique for a radical cation and which can best be described as solvent switching reactions : neutral reagents replace the acetyl radical in the ion. High-level ab initio molecular orbital calculations up to the MP2/6-31G**//6=31G**+ZPVE level of theory were performed to study the potential energy surface of the [C3H6O2].+ isomers. The hydrogen-bridged species 2 was found to be more stable by 10.2 kcal/mol than the covalently bonded, ionized hydroxyacetone 1. The enol isomers of ionized hydroxyacetone were calculated to be the most stable of all the structures studied, but these ions were not observed experimentally. The importance of three other hydrogen-bridged radical cations, [CH3C(H)=O...H...O=CH].+ (3), [CH2=C(H)O...H... O=CH2].+ (4), and [CH3C(O)...H...O=CH2].+ (5), is discussed.