Post-translational glycosylation of proteins play key roles in cellular processes and the site-specific characterisation of glycan structures is critical to understanding these events. Given the challenges regarding identification of glycan isomers, glycoproteomic studies generally rely on the assumption of conserved biosynthetic pathways. However, in a recent study, we found characteristically different HexNAc oxonium ion fragmentation patterns that depend on glycan structure. Such patterns could be used to distinguish between glycopeptide structural isomers. To acquire a mechanistic insight, deuterium-labelled glycopeptides were prepared and analysed. We found that the HexNAc-derived m/z 126 and 144 oxonium ions, differing in mass by H2 O, had completely different structures and that high-mannose N-glycopeptides generated abundant Hex-derived oxonium ions. We describe the oxonium ion decomposition mechanisms and the relative abundance of oxonium ions as a function of collision energy for a number of well-defined glycan structures, which provide important information for future glycoproteomic studies.