Combined chemical tagging followed by Endo-A catalysed elongation allows access to homogeneous, elaborated glycoproteins. A survey of different linkages and sugars demonstrated not only that unnatural linkages can be tolerated but they can provide insight into the scope of Endo-A transglycosylation activity. S-linked GlcNAc-glycoproteins are useful substrates for Endo-A extensions and display enhanced stability to hydrolysis at exposed sites. O-CH2-triazole-linked GlcNAc-glycoproteins derived from azidohomoalanine-tagged protein precursors were found to be optimal at sterically demanding sites.