The structures of the bound 13C/2H double-labelled 2'(R/S), 5'(R/S)-2H2-1',2',3',4',5'-13C5-2'-deoxyadenosine and the corresponding 2'-deoxycytidine moieties in the complexes with human deoxycytidine kinase (dCK) have been characterized for the first time by the solution NMR spectroscopy, using Transferred Dipole-Dipole Cross-correlated Relaxation and Transferred nOe experiments. It has been shown that the ligand adopts a South-type sugar conformation when bound to dCK.