Ionization and appearance potential measurements on aliphatic and ω- phenyl carbonyl compounds show that electron-impact fragmentation of these compounds α and β to the carbonyl group resulting in charge retention on the hydrocarbon fragment can occur without ionization of the carbonyl group. Structures are assigned to ionic and neutral fragments on the basis of their enthalpies, and evidence is presented favouring the elimination of the neutral enol forms of acetone and acetaldehyde in the hydrogen-rearranged β fission of ketones and aldehydes.