Several techniques and problems in determining the initial and final states of unimolecular dissociation processes are compared for neutral molecules, singly and doubly-charged cations. The general properties of doubly-charged ions relevant to state-to-state studies are presented and the photoion—photoion coincidence (PIPICO) technique for investigating their dissociative relaxation is described. Fragmentation of the CH 2+ 4 photoion over the excitation range 35–52 eV is examined in detail on the basis of previous PIPICO results and other relevant material. This leads to a comprehensive state-to-state interpretation of the observed dissociation channels of CH 2+ 4.