A series of investigations probing the mechanism of the 2,3-anhydrosugar migration-glycosylation reaction were performed using a thioglycoside with the D-lyxo stereochemistry as the substrate. Among the work reported are the results of quantum mechanical calculations, NMR studies, the measurement of a-deuterium kinetic isotope effects, and the synthesis of a series of substrate analogues. All studies point to a consistent finding: that the reaction proceeds through an oxocarbenium ion intermediate, not an episulfonium ion as previously suggested. It is proposed that the high stereoselectivity of the reaction arises from a preferred "inside attack" of the nucleophile onto the oxocarbenium ion intermediate.