Unequivocal evidence that the anomerization of acetylated alkyl glycopyranosides can proceed by way of an intramolecular mechanism was obtained through the observation that a racemic mixture of methyl β-glucopyranoside tetraacetate with theD-isomer labelled in the methoxyl group with carbon-14 is anomerized both by titanium tetrachloride and boron trifluoride without transfer of radioactive methoxyl groups to theL-isomer. It is submitted that these intramolecular anomerizations are best rationalized as the result of an unsuccessful attempt by the environment to bring about glycosidic cleavage and proceed by way of an ion-pair intermediate in which the anion is derived from the aglycon group.