Warfarin and acenocoumarol, anticoagulant drugs commonly used to prevent thrombosis and thromboembolism, are usually described in literature as open keto-enol tautomers, although research has shown that they also exist as two diastereomeric hemiacetal forms. This study aims to shed some light on this question by means of NMR experiments and theoretical calculations. The analysis of 1H NMR and 13C NMR spectra of warfarin and acenocoumarol (extracted from commercial formula) in DMSO-d6 reveals that there are two main species in solution. Two dimensional HSQC, COSY and HMBC spectra permitted the separation of signals in two sets, each one corresponding to one tautomer. A first approach reveals that these two species are the diastereomeric hemiacetals. NOESY experiments confirm that the two main species (hemiacetals) are in equilibrium with the minor keto-enol tautomer. DFT calculations were carried out on the three possible tautomers. Comparison between theoretical and experimental chemical shifts was made by means of DP4+ method, which in the case of warfarin assigned the major tautomer as the (R,R)-hemiacetal. In the case of acenocoumarol, DP4+ method failed to give a correct assignation and CP2 and CP3 methods were applied, suggesting that acenocoumarol exists mainly as the (R,R)-hemiacetal tautomer in DMSO.