The mass spectra of the trimethylsilyl ethers of the four positional isomers of methyl-O-palmitoyl-alpha-D-glucopyranoside have been studied, and the structures of the principal ions assigned by the use of exact mass measurements and deuterium labelling on the aliphatic chain, the trimethylsilyl group and the glucosidic methyl group. The origin of some fragments has been elucidated by analysis of reactions of metastable ions. The mass spectra of the different isomers exhibit major differences, which depend upon the presence or absence of the aliphatic chain. These results indicate that this is a useful method for determining the position of an acyl group in a methyl glucoside. The applicability of this mass spectrometric method in structural determination of unknown acylated sugars is discussed. The structure of a corynomycoloyl-alpha-D-trehalose, isolated from Corynebacterium diphtheriae, has been determined by mass spectrometry as an application of the method. The molecular weight of this compound has been determined by field desorption mass spectrometry (cationization method), and the study of the electron impact spectrum of the trimethylsilyl derivatives clearly demonstrates that the corynomycolic acid is linked by an ester group to position 6 of the trehalose molecule.