4-Oxoglycosides of xylo-configuration (1a, 1b) rearranged in pyridine-d₅ to give exclusively 3-oxoglycosides of ribo-configuration through enediol intermediates. On the other hand, 4-oxoglycosides of lyxo- and arabino-configuration (5, 6) gave an epimeric mixture of both isomers. The reason for this difference was discussed on the basis of AM1 calculations.