Stereoisomerically pure samples of (2R)- and (2S)-2-hydroxypropyl-, (2R)- and (2S)-2,3-dihydroxypropyl-, (3S)-3,4-dihydroxybutyl-, and (4S)-4,5-dihydroxypentyl-cobalamin have been synthesised by allowing cob(I) alamin to react with the appropriate optically pure alkylating agent; these alkylcobalamins and the mixtures of diastereoisomers obtained by allowing cob(I) alamin to react with the corresponding racemic alkylating agents have been characterised by high field 1H n.m.r. spectroscopy.