The present study demonstrates that matrix-assisted laser desorption ionization/postsource decay (MALDI/PSD) analysis of the molecular cation of glycopeptides derivatized at their amino terminus with a phosphonium group cleaves peptide backbone without removing the glycan. The predictable a-type fragment ions retain the glycan moiety, enabling unambiguous localization of O-glycans on the peptide chain. In contrast, collision-activated dissociation tandem mass spectrometry analysis carried out on the doubly charged protonated phosphonium cation results in the predominant loss of the sugar moiety from the peptide. This result supports the previously proposed charge-induced fragmentation mechanism of the sugar-peptide bond. MALDI/PSD analysis of glycopeptides converted to their acetyl phosphonium derivatives is an effective alternative to electron capture dissociation, as illustrated by the positioning of up to three GalNac residues along the full tandem repeat peptide sequence derived from the MUC 5AC mucin.