(5′R)‐2H1)Adenosine [(5′R):(5′S) = 85:15] was prepared by a procedure which involved inter alia the reduction of 6‐N‐benzoyl‐2′,3′‐O‐isopropylidene 5′‐oxoadenosine with a reagent obtained from LiAl2H4 and (‐)‐ isoborneol. (5′S)‐(5′‐2H1)AdoCbl= 5′S):(5′R) = 74:26] (AdoCbl = 5′ deoxyadenosylcoalamin) was synthesised by reacting cobal(I)amin with (5′R)‐2′‐3′‐O‐isopropylidene‐5′‐tosyl‐(5′‐2H1) adenosine followed by acid hydrolysis to remove the isopropylidene protective group. (5′R)‐2H1)AdpCbl [(5′S):(5′R) = 77:23] was prepared by reacting cobal (I) amin with (5′S)‐5′‐chloro 5′‐(5′‐2H1) deoxyadenosine [(5′S):(5′R= 80:20] obtained in turn from (5′R)‐(5′‐2H1)adenosine. The reaction sequence involved two consecutive inversions at the C‐5′ atom of adenosine. Comparison of the 500‐MHz 1H‐NMR spectra of unlaelled, (5′S)‐ and (5′R)‐(5′2H1)AdCbl allowed assignment of the triplet at 0.58 ppm and the doublet at 1.525 ppm to the diastereotopic 5′‐HRe and 5′‐HSt atoms, respectively. On acidification, these two protons gave rise to two triplets at 0.11 ppm and 1.78 ppm indicating that torsion had occurred around the C‐4′‐C‐5′ bond. Samples of (5′R)‐ and (5′S)‐(5′2H1)AdoCbl were incubated with metylmalonyl0‐CoA mutase from Propioniacterium shermanii Examination by 1H‐NMR spectroscopy at 500 MHz revealed partial los and stereochemical scrambling of the deuterium at the 5′ position. This indicates transient conversion of the C‐5′ atom into a torsiosymmetric group and hence cleavage of the cobalt‐carbon bond during interaction with the enzyme. The mechanism by which deuterium is lost remains to be elucidated.