The O-methyl phosphoramidate (MeOPN) motif is anon-stoichiometric modification of capsular polysaccharides (CPS) in % 70 %ofall Campylobacter jejuni strains.Infections by C. jejuni lead to food-borne illnesses and the CPS they produce are key virulence factors.T he MeOPN phosphorus atom in these CPS is stereogenic and is found as as ingle stereoisomer.However,todate,the absolute stereochemistry at this atom has been undefined. We report the synthesis of the three repeating units found in C. jejuni 81-176 CPS;o ne of these possesses aMeOPN group.Inthe course of these studies we established that the stereochemistry of the phosphorus atom in this MeOPN group is R. These studies represent the first unequivocal proof of stereochemistry of this group in any C. jejuni CPS.T he compounds produced are anticipated to be useful tools in investigations targeting the function and biosynthesis of this structurally-interesting modification, which so far has only been identified in campylobacter.