The conformation of 4-thiocellobiose bound to beta-glucosidase from Streptomyces sp. has been studied by 1H-NMR transferred nuclear Overhauser effect spectroscopy (TR-NOE). Thiocellobiose behaves as an inhibitor of this glucosidase when cellobiose is used as substrate. NOE measurements and molecular mechanics calculations have also been performed to estimate the probability distribution of conformers of thiocellobiose when free in solution. Experimental data show that, in contrast with the natural O-analogue, thiocellobiose presents three conformational families in the free state, namely syn, anti-psi and anti-phi, whilst only one of them (syn) is recognized by the enzyme.