Arabinogalactan (AG) and sulphated arabinogalactans (SAG) which are products of chemical modification of arabinogalactan polysaccharide with anticoagulant properties were studied by experimental infrared (IR) spectroscopy combined with density functional theory simulations. Mutual analythis of experimental and theoretical IR frequencies indicates that the descripancies between experiment and theory is caused by the influence of -OH groups, which led to the energy shift and broadening of the absorption IR bands. It was found that theoretical and experimental spectra correspond well within 3000-4000 cm -1 spectral region. Addition of sulphur group in AG structure causes hydroxyl group to become accessible for further sulphation. The difference between experimental and theoretical IR frequencies of sulphated AG derivatives is greater than for the parent arabinogalactan due to increase in the number of possible isomers and conformers.