Uronic acids are important constituents of polysaccharides found on the cell membranes of different organisms. To prepare uronic acid containing oligosaccharides, uronic acid 6,3-lactones can be employed as they display a fixed conformation and a unique reactivity and stereoselectivity. The mechanism of glycosylation of uronic acid 6,3-lactones and derivatives thereof is established using a combination of techniques including IR ion spectroscopy combined with quantum-chemical calculations and variable-temperature nuclear magnetic resonance (VT NMR) spectroscopy. The role of these intermediates in glycosylation is assayed by varying the activation protocol and acceptor nucleophilicity. The observed trends show analogy to the well-studied 4,6-benzylidene glycosides and may be used to guide the development of next-generation stereoselective glycosyl donors.