IN the course of a study of the mechanisms involved in the solvolytic reactions of glycosides, we have investigated the position of bond fission occurring in the hydrolysis of α- and β-D-methylglucosides. The reactions were carried out in water enriched in oxygen-18, and the methanol produced was isolated by fractional distillation in an efficient column, and then pyrolysed to give carbon monoxide which was examined by a mass spectrometer. We have found that the hydrolysis of α- and β-D-methylglucosides both in 1 N acid solution and in the presence of appropriate methylglucosidases proceeds by fission of the hexose–oxygen bond: