The ab initio architecture and torsional barrier for acetone n-radical cation are obtained. The 923 cm(-1) MP4/6-311+G(3df,2p) barrier is calculated to be 30% higher than for neutral acetone. This increase is largely attributed to correlation effects and less importantly to increased hyperconjugative stabilization of the equilibrium cation conformer. Ionization is predicted to cause opening of the central CCC angle by 7degrees and cause the methyl groups to lose the C-3v symmetry that they possess in neutral acetone. The torsional coordinate for the infrared active b(1) (gearing) rotation is predicted to not lie purely on the torsional potential surface, but to be contaminated by puckering of the CCCO skeleton in both the neutral and cation species, thereby making the b(1) infrared torsional frequencies only partially suitable for sampling the torsional potential surface.