The rate constants for the hydrolysis of some al-kyl α- and β-d-fructofuranosides in aqueous perchloric acid have been determined at various temperatures. The effects of varying the aglycon structure on the hydrolysis rate suggest, together with the markedly positive entropies of activation, that the substrate, protonated on the glycosidic oxygen atom, undergoes a rate-limiting unimolecular heterolysis to form a glycosyl oxocar-benium ion. The rate variations in mixtures of aqueous perchloric acid and dimethyl sulfoxide are interpreted as lending further support for the proposed mechanism.