Fully protected diastereoisomers of deoxyguanylyl(3′ → 5′)deoxyadenosine stereospecifically labelled on phosphorus with oxygen-18 have been synthesized by oxidation of phosphite triester intermediates in the presence of 18 O-labelled water. The diastereoisomers have been chromatographically separated and their absolute configuration at phosphorus determined. (R p )-[ 18 o]deoxyguanylyl(3′ → 5′) deoxyadenosine has been prepared by complete deprotection of the parent diastereoisomer of the S p configuration. Methylation of the former compound permits assignment of the absolute configurations of the methyl esters of N 1 -methyldeoxyguanylyl(3′ → 5′)N 1 -methyldeoxyadenosine.