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Some partially methylated methyl glucosides were investigated by the gas chromatography-mass spectrometry method. The main properties of their mass spectra can be derived from the fragmentation scheme of permethylated pyranoses by a consideration of the effects of the different substituents. Apart from a shift of the ion masses, due to replacement of methoxyl by hydroxyl groups, the main effect of the substituent is a variation of the inductive stabilisation of the positive charge of the fragments. Furthermore, the derivatives having methoxyl groups at the 1,2,3-, 1,2,3,6-, 1,3,4-, or 1,3,4,6-positions produce a very characteristic peak at m/e 161. The stereochemistry of the partially methylated methyl glucosides has no significant effect on their mass spectra, but the effect of the position of the substituent is important. The type of substitution can thence be deduced from the mass spectra. The gas chromatography-mass spectrometry method is much more appropriate for the identification of a mixture of monosaccharides obtained from a permethylated polysaccharide by solvolysis than the gas-chromatographic method alone.