Two parallel unimolecular dissociation channels of the acetone cation radical are known experimentally. Methyl loss is the dominant reaction channel for high internal energies of the reactant ion whereas methane elimination is the major reaction channel for low internal energies. However, there exists a controversy about the mechanisms of dissociation of acetone cation radical. High level ab initio MP3/6-311G(d, p)//MP3/6-311G(d,) calculations have been carried out to investigate the most relevant parts of the C3H6O+·ground state potential energy surface. Reactant and transition state frequencies also have been obtained at the MP3/6-311G(d, p) level to include zero-point energy effects. These energy results reinforce the idea that methane elimination at low internal energies may take place only via a tunnelling mechanism through the hydrogen transfer energy barrier. To analyse the influence of the entropic contribution on the mechanism the Gibbs free energy was calculated. Variational calculations at the hydrogen transfer region indicate that conventional transition state theory correctly describes the main features of the system at that bottleneck. © 1997 Taylor & Francis Group, LLC.