.3,4,6-Tri-0-benzyl-D-mannose and methyl 3,4,6-tri-0-benzyl-a-D-mannopyranoside were subjected to fluorination by diethylaminosulfur trifluoride and each produced two products. The former suffered fluorination at the anomeric centre to give the a-fluoride and, to a lesser extent, additionally at C-2 to give the 2-fluoro-2-deoxy-P-D-glucopyranosyl fluoride. The methyl mannoside underwent a methyl ether migration reaction to produce a 1 : 1 mixture of the a-and P-2-0-methyl glucosyl fluorides.A mechanism for this rearrangement is proposed and 'H and ' % nmr data are presented for all products. [Traduit par la revue]