Two-dimensional mass spectrometry (2D MS) correlates precursor and fragment ions without ion isolation in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR MS) for tandem mass spectrometry. Infrared activated electron capture dissociation (IR-ECD), using a hollow cathode configuration, generally yields more information for peptide sequencing in tandem mass spectrometry than ECD (electron capture dissociation) alone. The effects of the fragmentation zone on the 2D mass spectrum are investigated as well as the structural information that can be derived from it. The enhanced structural information gathered from the 2D mass spectrum is discussed in terms of how de novo peptide sequencing can be performed with increased confidence. 2D IR-ECD MS is shown to sequence peptides, to distinguish between leucine and isoleucine residues through the production of w ions as well as between C-terminal ( b/ c) and N-terminal ( y/ z) fragments through the use of higher harmonics, and to assign and locate peptide modifications.