The α-carbomethoxy carbenium ions CH 3OC(O)C +R 1R 2 where R 1 = R 2 = H ( 1 +, R 1 H, R 2 CH 3 ( 2 +), R 1 R 2 CH 3 ( 3 +) and R 1 H, R 2 C 6H 5 ( 4 +) have been prepared by collisional charge inversion and by neutralisation-reionisation of the appropriate enolate ions. In addition, the α-benzoyl carbenium ions C 6H 5C(O)C +R 1R 2 where R 1 R 2 H ( 5 +), R 1 H, R 2 CH 3 ( 6 +) and R 1 R 2 CH 3 ( 7 +) have been prepared by the same methods. In all cases the charge inversion and neutralisation-reionisation mass spectra are very similar. For the α-carbomethoxy carbenium ions the major fragmentation reaction involves elimination of CH 3O & to form the appropriate ketene molecular ion. The spectra are quite different from those obtained by collision-induced dissociation of α-carbomethoxy carbenium ions prepared by electron ionisation. The ions formed by charge inversion fragments to a much lesser extent by rearrangement, presumably because they have a higher mean internal energy. For the α-benzoyl carbenium ions formation of the ketene molecular ion is negligible, a major reaction channel being elimination fo CR 1R 2 to form the benzoyl ion. A recovered positive ion signal at the same m/z value as the enolate precursor is observed only for 3 + and 4 +, indicating that only the most stable of the destabilised carbenium ions will be observed as intact species when formed by charge inversion of negative ions.