A new tandem mass spectrometry technique, electron ionization dissociation (EID), employs irradiation of trapped cations [M + nH](n+) (n >= 1) by fast electrons with energy at least 10 eV higher than the ionization threshold of the cations. Such irradiation causes simultaneous ionization and electronic excitation of the irradiated species, which is equivalent of double-ionization to [M + nH]((n+2)+) followed by electron capture to form electronically excited [M + nH]((n+1)+) (center dot) ions. Subsequent fragmentation of the latter species gives both side-chain losses and backbone fragmentation. Theoretical efficiency of such fragmentation calculated as a ratio of the fragment ion current and the precursor ion depletion can reach (n+1)/n, that is, can exceed 100%. EID often leads to backbone N-C-alpha bond cleavage, giving c-/Z-type fragments. C-C bond cleavage, giving preferentially a-/x-type fragments, is also observed. EID could be used in bottom-up proteomics for both electrospray and matrix-assisted laser desorption ionization (MALDI) -produced ions. The energy incorporated into the precursor ion in a single interaction with electrons can exceed 10 eV, which makes EID suitable for top-down analysis of folded gas-phase protein conformations.