Abstract The action of potassium hydrogen fluoride (KHF 2) in ethane-1,2-diol on methyl 2,3-anhydro-4- O-benzyl-β- D-ribopyranoside ( 1) and methyl 2,3-anhydro-5- O-benzyl-α- D-lyxofuranoside ( 7) has been examined. In both cases the expected trans scission of the epoxide ring occurs to yield methyl 4- O-benzyl-3-deoxy-3-fluoro-β- D-xylopyranoside ( 2) and methyl 5- O-benzyl-3-deoxy-3-fluoro-α- D-arabinofuranoside ( 8) respectively. The structure of compound ( 2) was established by catalytic hydrogenation, which removed the benzyl group, to yield a crystalline glycoside ( 3) which did not consume periodate. Acid hydrolysis of methyl 3-deoxy-3-fluoro-β- D-xylopyranoside ( 3) yielded syrupy 3-deoxy-3-fluoro-αβ- D-xylose ( 5) as a syrup characterized as the 2,5-dichlorophenylhydrazone. Acid hydrolysis of compound ( 8) yielded a reducing sugar ( 9), characterised as the phenylhydrazone. Compound ( 9) consumed 1 mol. of periodate and liberated formic acid which is consistent with the structure 5- O-benzyl-3-deoxy-3-fluoro-αβ- D-arabinose. Catalytic hydrogenation of the fluorohydrin ( 8) removed the benzyl group to give methyl 3-deoxy-3-fluoro-α- D-arabinofuranoside ( 10) which, on acid hydrolysis, yielded crystalline 3-deoxy-3-fluoro-β- D-arabinose ( 13). The β-configuration was assigned to compound ( 13) on the basis of its mutarotation. This method of introducing fluorine into carbohydrates affords reasonable yields of uncontaminated fluorohydrins and is considered to have general applicability.