A four-sector mass spectrometer was employed to study fragments formed by the high-energy collision of neutral peptides with molecular oxygen. Neutral peptides were generated in the gas phase from proton-bound dimers by the method of Wesdemiotis and co-workers [Anal Chem. 65, 1594 (1993)]. B/E scanning was used in MS-2 to collected the fragment ions from neutral precursors. Molecular ions (radical ions) were not obtained in most of the spectra, but a variety of fragment ions were recorded. High resolution and mass accuracy allowed most fragment ions to be assigned. Most fragment ions are of the same types as those produced from protonated peptides by collisional activation. Formation of y-type ions is significantly reduced in the processes induced by collisional ionization, compared with high- and low-energy collisional activation of protonated peptide precursors.